Added a selection model, and attribute selection, and cleaned up foreign key referencing.

src/main/java/nl/andrewlalis/erme/control/actions/ExportToImageAction.java

@@ -124,10 +124,10 @@ public class ExportToImageAction extends DiagramPanelAction {
 		// Render the model.
 		boolean lolcat = model.isLolcatEnabled(); // save previous lolcat mode
 		model.setLolcatEnabled(false);
-		List<Relation> selectedRelations = model.getSelectedRelations();
+		List<Relation> selectedRelations = model.getSelectionModel().getSelectedRelations();
-		model.getSelectedRelations().forEach(r -> r.setSelected(false));
+		model.getSelectionModel().clearSelection();
 		new MappingModelViewModel(model).draw(g2d);
-		model.getRelations().forEach(r -> r.setSelected(selectedRelations.contains(r)));
+		model.getSelectionModel().selectAll(selectedRelations);
 		model.setLolcatEnabled(lolcat); // revert previous lolcat mode
 
 		// Revert to the normal image space, and render a watermark.

src/main/java/nl/andrewlalis/erme/control/actions/LoadSampleModelAction.java

@@ -1,6 +1,9 @@
 package nl.andrewlalis.erme.control.actions;
 
-import nl.andrewlalis.erme.model.*;
+import nl.andrewlalis.erme.model.Attribute;
+import nl.andrewlalis.erme.model.AttributeType;
+import nl.andrewlalis.erme.model.MappingModel;
+import nl.andrewlalis.erme.model.Relation;
 import nl.andrewlalis.erme.view.DiagramPanel;
 
 import java.awt.*;
@@ -22,7 +25,9 @@ public class LoadSampleModelAction extends DiagramPanelAction {
 		Relation r1 = new Relation(model, new Point(50, 100), "Airplane");
 		r1.addAttribute(new Attribute(r1, AttributeType.ID_KEY, "id"));
 		r1.addAttribute(new Attribute(r1, AttributeType.PLAIN, "purchasedAt"));
-		r1.addAttribute(new ForeignKeyAttribute(r1, AttributeType.PLAIN, "typeName", "AirplaneType", "name"));
+		Attribute fk = new Attribute(r1, AttributeType.PLAIN, "typeName");
+		fk.setReference(model.findAttribute("AirplaneType", "name"));
+		r1.addAttribute(fk);
 		model.addRelation(r1);
 		getDiagramPanel().setModel(model);
 	}

src/main/java/nl/andrewlalis/erme/control/actions/edits/AddAttributeAction.java

@@ -3,6 +3,7 @@ package nl.andrewlalis.erme.control.actions.edits;
 import nl.andrewlalis.erme.control.actions.DiagramPanelAction;
 import nl.andrewlalis.erme.model.*;
 import nl.andrewlalis.erme.view.DiagramPanel;
+import nl.andrewlalis.erme.view.EditAttributePopupDialog;
 
 import javax.swing.*;
 import java.awt.*;
@@ -23,7 +24,7 @@ public class AddAttributeAction extends DiagramPanelAction {
 	public void actionPerformed(ActionEvent e) {
 		DiagramPanel dp = getDiagramPanel();
 		MappingModel model = dp.getModel();
-		List<Relation> selectedRelations = model.getSelectedRelations();
+		List<Relation> selectedRelations = model.getSelectionModel().getSelectedRelations();
 		if (selectedRelations.size() != 1) {
 			JOptionPane.showMessageDialog(
 					dp,
@@ -34,72 +35,74 @@ public class AddAttributeAction extends DiagramPanelAction {
 			return;
 		}
 		Relation r = selectedRelations.get(0);
-		Attribute createdAttribute;
+		EditAttributePopupDialog popup = new EditAttributePopupDialog((JFrame) SwingUtilities.getWindowAncestor(dp), r, null);
-		String name = JOptionPane.showInputDialog(dp, "Enter the name of the attribute.", "Attribute Name", JOptionPane.PLAIN_MESSAGE);
+		popup.setVisible(true);
-		if (name == null) return;
+//		Attribute createdAttribute;
-		Integer index = (Integer) JOptionPane.showInputDialog(
+//		String name = JOptionPane.showInputDialog(dp, "Enter the name of the attribute.", "Attribute Name", JOptionPane.PLAIN_MESSAGE);
-				dp,
+//		if (name == null) return;
-				"Select the index to insert this attribute at.",
+//		Integer index = (Integer) JOptionPane.showInputDialog(
-				"Attribute Index",
+//				dp,
-				JOptionPane.PLAIN_MESSAGE,
+//				"Select the index to insert this attribute at.",
-				null,
+//				"Attribute Index",
-				Stream.iterate(0, n -> n + 1).limit(r.getAttributes().size() + 1).toArray(),
+//				JOptionPane.PLAIN_MESSAGE,
-				r.getAttributes().size()
+//				null,
-		);
+//				Stream.iterate(0, n -> n + 1).limit(r.getAttributes().size() + 1).toArray(),
-		if (index == null) return;
+//				r.getAttributes().size()
-		AttributeType type = (AttributeType) JOptionPane.showInputDialog(
+//		);
-				dp,
+//		if (index == null) return;
-				"Select the type this attribute is.",
+//		AttributeType type = (AttributeType) JOptionPane.showInputDialog(
-				"Attribute Type",
+//				dp,
-				JOptionPane.PLAIN_MESSAGE,
+//				"Select the type this attribute is.",
-				null,
+//				"Attribute Type",
-				AttributeType.values(),
+//				JOptionPane.PLAIN_MESSAGE,
-				AttributeType.PLAIN
+//				null,
-		);
+//				AttributeType.values(),
-		if (type == null) return;
+//				AttributeType.PLAIN
-		boolean shouldUseForeignKey = ((String) JOptionPane.showInputDialog(
+//		);
-				dp,
+//		if (type == null) return;
-				"Is this attribute a foreign key?",
+//		boolean shouldUseForeignKey = ((String) JOptionPane.showInputDialog(
-				"Foreign Key",
+//				dp,
-				JOptionPane.PLAIN_MESSAGE,
+//				"Is this attribute a foreign key?",
-				null,
+//				"Foreign Key",
-				new String[]{"Yes", "No"},
+//				JOptionPane.PLAIN_MESSAGE,
-				"No"
+//				null,
-		)).equalsIgnoreCase("yes");
+//				new String[]{"Yes", "No"},
-		if (shouldUseForeignKey) {
+//				"No"
-			if (model.getRelations().size() < 2) {
+//		)).equalsIgnoreCase("yes");
-				JOptionPane.showMessageDialog(dp, "There should be at least 2 relations present in the model.", "Not Enough Relations", JOptionPane.WARNING_MESSAGE);
+//		if (shouldUseForeignKey) {
-				return;
+//			if (model.getRelations().size() < 2) {
-			}
+//				JOptionPane.showMessageDialog(dp, "There should be at least 2 relations present in the model.", "Not Enough Relations", JOptionPane.WARNING_MESSAGE);
-			Relation fkRelation = (Relation) JOptionPane.showInputDialog(
+//				return;
-					dp,
+//			}
-					"Select the relation that this foreign key references.",
+//			Relation fkRelation = (Relation) JOptionPane.showInputDialog(
-					"Foreign Key Relation Reference",
+//					dp,
-					JOptionPane.PLAIN_MESSAGE,
+//					"Select the relation that this foreign key references.",
-					null,
+//					"Foreign Key Relation Reference",
-					model.getRelations().toArray(new Relation[0]),
+//					JOptionPane.PLAIN_MESSAGE,
-					model.getRelations().stream().findFirst().orElse(null)
+//					null,
-			);
+//					model.getRelations().toArray(new Relation[0]),
-			if (fkRelation == null) return;
+//					model.getRelations().stream().findFirst().orElse(null)
-			List<Attribute> eligibleAttributes = fkRelation.getAttributes();
+//			);
-			if (eligibleAttributes.isEmpty()) {
+//			if (fkRelation == null) return;
-				JOptionPane.showMessageDialog(dp, "There are no referencable attributes in the selected relation.", "No Referencable Attributes", JOptionPane.WARNING_MESSAGE);
+//			List<Attribute> eligibleAttributes = fkRelation.getAttributes();
-				return;
+//			if (eligibleAttributes.isEmpty()) {
-			}
+//				JOptionPane.showMessageDialog(dp, "There are no referencable attributes in the selected relation.", "No Referencable Attributes", JOptionPane.WARNING_MESSAGE);
-			Attribute fkAttribute = (Attribute) JOptionPane.showInputDialog(
+//				return;
-					dp,
+//			}
-					"Select the attribute that this foreign key references.",
+//			Attribute fkAttribute = (Attribute) JOptionPane.showInputDialog(
-					"Foreign Key Attribute Reference",
+//					dp,
-					JOptionPane.PLAIN_MESSAGE,
+//					"Select the attribute that this foreign key references.",
-					null,
+//					"Foreign Key Attribute Reference",
-					eligibleAttributes.toArray(new Attribute[0]),
+//					JOptionPane.PLAIN_MESSAGE,
-					eligibleAttributes.get(0)
+//					null,
-			);
+//					eligibleAttributes.toArray(new Attribute[0]),
-			if (fkAttribute == null) return;
+//					eligibleAttributes.get(0)
-			createdAttribute = new ForeignKeyAttribute(r, type, name, fkAttribute);
+//			);
-		} else {
+//			if (fkAttribute == null) return;
-			createdAttribute = new Attribute(r, type, name);
+//			createdAttribute = new ForeignKeyAttribute(r, type, name, fkAttribute);
-		}
+//		} else {
-		r.addAttribute(createdAttribute, index);
+//			createdAttribute = new Attribute(r, type, name);
+//		}
+//		r.addAttribute(createdAttribute, index);
 	}
 }

src/main/java/nl/andrewlalis/erme/control/actions/edits/AddRelationAction.java

@@ -43,9 +43,9 @@ public class AddRelationAction extends DiagramPanelAction {
 				p = new Point(bounds.x + bounds.width / 2, bounds.y + bounds.height / 2);
 			}
 			Relation r = new Relation(model, p, name);
-			model.getSelectedRelations().forEach(rl -> rl.setSelected(false));
-			r.setSelected(true);
 			model.addRelation(r);
+			model.getSelectionModel().clearSelection();
+			model.getSelectionModel().select(r);
 			if (isFirstRelation) {
 				model.normalizeRelationPositions();
 				dp.centerModel();

src/main/java/nl/andrewlalis/erme/control/actions/edits/RemoveAttributeAction.java

@@ -2,7 +2,6 @@ package nl.andrewlalis.erme.control.actions.edits;
 
 import nl.andrewlalis.erme.control.actions.DiagramPanelAction;
 import nl.andrewlalis.erme.model.Attribute;
-import nl.andrewlalis.erme.model.Relation;
 import nl.andrewlalis.erme.view.DiagramPanel;
 
 import javax.swing.*;
@@ -20,28 +19,22 @@ public class RemoveAttributeAction extends DiagramPanelAction {
 
 	@Override
 	public void actionPerformed(ActionEvent e) {
-		List<Relation> selectedRelations = getDiagramPanel().getModel().getSelectedRelations();
+		List<Attribute> selectedAttributes = getDiagramPanel().getModel().getSelectionModel().getSelectedAttributes();
-		if (selectedRelations.size() != 1 || selectedRelations.get(0).getAttributes().isEmpty()) {
+		if (selectedAttributes.isEmpty()) {
 			JOptionPane.showMessageDialog(
 					getDiagramPanel(),
-					"A single relation with at least one attribute must be selected to remove an attribute.",
+					"At least one attribute must be selected to remove.",
-					"Single Relation With Attribute Required",
+					"Select Attributes to Remove Them",
 					JOptionPane.WARNING_MESSAGE
 			);
 			return;
 		}
-		Relation r = selectedRelations.get(0);
+		int choice = JOptionPane.showConfirmDialog(getDiagramPanel(), "Are you sure you want to remove these attributes?", "Confirm", JOptionPane.OK_CANCEL_OPTION);
-		Attribute attribute = (Attribute) JOptionPane.showInputDialog(
+		if (choice == JOptionPane.YES_OPTION) {
-				getDiagramPanel(),
+			for (Attribute a : selectedAttributes) {
-				"Select the attribute to remove.",
+				a.getRelation().removeAttribute(a);
-				"Select Attribute",
+			}
-				JOptionPane.PLAIN_MESSAGE,
+			getDiagramPanel().getModel().getSelectionModel().clearAttributes();
-				null,
-				r.getAttributes().toArray(new Attribute[0]),
-				r.getAttributes().get(0)
-		);
-		if (attribute != null) {
-			r.removeAttribute(attribute);
 		}
 	}
 }

src/main/java/nl/andrewlalis/erme/control/actions/edits/RemoveRelationAction.java

@@ -9,6 +9,7 @@ import javax.swing.*;
 import java.awt.event.ActionEvent;
 import java.awt.event.InputEvent;
 import java.awt.event.KeyEvent;
+import java.util.List;
 
 public class RemoveRelationAction extends DiagramPanelAction {
 	public RemoveRelationAction(DiagramPanel diagramPanel) {
@@ -20,7 +21,8 @@ public class RemoveRelationAction extends DiagramPanelAction {
 	@Override
 	public void actionPerformed(ActionEvent e) {
 		MappingModel model = getDiagramPanel().getModel();
-		if (model.getSelectedRelations().isEmpty()) {
+		List<Relation> selectedRelations = model.getSelectionModel().getSelectedRelations();
+		if (selectedRelations.isEmpty()) {
 			JOptionPane.showMessageDialog(
 					getDiagramPanel(),
 					"No relations selected. Select at least one relation to remove.",
@@ -29,8 +31,12 @@ public class RemoveRelationAction extends DiagramPanelAction {
 			);
 			return;
 		}
-		for (Relation r : model.getSelectedRelations()) {
+		int choice = JOptionPane.showConfirmDialog(getDiagramPanel(), "Are you sure you want to remove these relations?", "Confirm", JOptionPane.YES_NO_OPTION);
-			model.removeRelation(r);
+		if (choice == JOptionPane.YES_OPTION) {
+			for (Relation r : selectedRelations) {
+				model.removeRelation(r);
+			}
+			model.getSelectionModel().clearRelations();
 		}
 	}
 }

src/main/java/nl/andrewlalis/erme/control/diagram/DiagramMouseListener.java

@@ -1,5 +1,6 @@
 package nl.andrewlalis.erme.control.diagram;
 
+import nl.andrewlalis.erme.model.Attribute;
 import nl.andrewlalis.erme.model.MappingModel;
 import nl.andrewlalis.erme.model.Relation;
 import nl.andrewlalis.erme.view.DiagramPanel;
@@ -25,7 +26,7 @@ public class DiagramMouseListener extends MouseAdapter {
 	 * - If the click occurs within at least one relation, select the first one,
 	 * and deselect all others if CTRL is not held down.
 	 * - If the user did a right-click, try to open a popup menu with some
-	 * possible actions.
+	 * possible actions, based on what entity we're over.
 	 * @param e The mouse event.
 	 */
 	@Override
@@ -40,15 +41,24 @@ public class DiagramMouseListener extends MouseAdapter {
 		final boolean isShiftDown = (e.getModifiers() & ActionEvent.SHIFT_MASK) == ActionEvent.SHIFT_MASK;
 		MappingModel model = this.diagramPanel.getModel();
 
-		if (!isShiftDown && !isCtrlDown) {// A simple click anywhere should reset selection.
+		if (!isShiftDown && !isCtrlDown) {// A simple left-click anywhere should reset selection.
-			model.getRelations().forEach(r -> r.setSelected(false));
+			model.getSelectionModel().clearSelection();
 		}
 
 		if (!isShiftDown) {// If the user clicked or CTRL+clicked, try and select the relation they clicked on.
 			for (Relation r : model.getRelations()) {
 				if (r.getViewModel().getBounds(g).contains(modelX, modelY)) {
-					r.setSelected(!r.isSelected());
+					boolean anyAttributeSelected = false;
-					break;
+					for (Attribute a : r.getAttributes()) {
+						if (a.getViewModel().getBounds(g).contains(modelX, modelY)) {
+							model.getSelectionModel().toggle(a);
+						}
+						anyAttributeSelected = anyAttributeSelected || a.isSelected();
+					}
+					if (!anyAttributeSelected) {
+						model.getSelectionModel().toggle(r);
+						break;
+					}
 				}
 			}
 		}
@@ -77,12 +87,11 @@ public class DiagramMouseListener extends MouseAdapter {
 		MappingModel model = this.diagramPanel.getModel();
 
 		if (isShiftDown) {
-			System.out.println(e);
 			this.diagramPanel.translate(-dx, -dy);
 			this.diagramPanel.repaint();
 		} else {
 			for (Relation r : model.getRelations()) {
-				if (r.isSelected()) {
+				if (model.getSelectionModel().isSelected(r)) {
 					r.setPosition(new Point(r.getPosition().x - dx, r.getPosition().y - dy));
 					changed = true;
 				}

src/main/java/nl/andrewlalis/erme/model/Attribute.java

@@ -13,6 +13,7 @@ public class Attribute {
 	private final Relation relation;
 	private AttributeType type;
 	private String name;
+	private Attribute reference;
 
 	private transient AttributeViewModel viewModel;
 
@@ -20,6 +21,7 @@ public class Attribute {
 		this.relation = relation;
 		this.type = type;
 		this.name = name;
+		this.reference = null;
 	}
 
 	public void setType(AttributeType type) {
@@ -32,6 +34,14 @@ public class Attribute {
 		this.relation.getModel().fireChangedEvent();
 	}
 
+	public boolean hasReference() {
+		return this.reference != null;
+	}
+
+	public void setReference(Attribute attribute) {
+		this.reference = attribute;
+	}
+	
 	public AttributeViewModel getViewModel() {
 		if (this.viewModel == null) {
 			this.viewModel = new AttributeViewModel(this);
@@ -39,6 +49,10 @@ public class Attribute {
 		return this.viewModel;
 	}
 
+	public boolean isSelected() {
+		return this.relation.getModel().getSelectionModel().isSelected(this);
+	}
+
 	@Override
 	public boolean equals(Object o) {
 		if (this == o) return true;

src/main/java/nl/andrewlalis/erme/model/ForeignKeyAttribute.java

@@ -1,39 +0,0 @@
-package nl.andrewlalis.erme.model;
-
-import lombok.Getter;
-
-@Getter
-public class ForeignKeyAttribute extends Attribute {
-	private Attribute reference;
-
-	public ForeignKeyAttribute(Relation relation, AttributeType type, String name, Attribute reference) {
-		super(relation, type, name);
-		this.reference = reference;
-	}
-
-	public ForeignKeyAttribute(Relation relation, AttributeType type, String name, String referencedRelationName, String referencedAttributeName) {
-		this(relation, type, name, relation.getModel().findAttribute(referencedRelationName, referencedAttributeName));
-		if (this.getReference() == null) {
-			throw new IllegalArgumentException("Unknown attribute name.");
-		}
-	}
-
-	public void setReference(Attribute reference) {
-		this.reference = reference;
-		this.getRelation().getModel().fireChangedEvent();
-	}
-
-	public String getFullReferenceName() {
-		return this.getReference().getRelation().getName() + "." + this.getReference().getName();
-	}
-
-	@Override
-	public String toString() {
-		return super.toString() + "->" + this.getFullReferenceName();
-	}
-
-	@Override
-	public ForeignKeyAttribute copy(Relation newRelation) {
-		return new ForeignKeyAttribute(newRelation, this.getType(), this.getName(), this.getReference());
-	}
-}

src/main/java/nl/andrewlalis/erme/model/MappingModel.java

@@ -10,9 +10,7 @@ import nl.andrewlalis.erme.view.view_models.MappingModelViewModel;
 import nl.andrewlalis.erme.view.view_models.ViewModel;
 
 import java.awt.*;
-import java.util.List;
 import java.util.*;
-import java.util.stream.Collectors;
 
 /**
  * This model contains all the information about a single mapping diagram,
@@ -33,10 +31,13 @@ public class MappingModel implements Viewable {
 	@Getter
 	@Setter
 	private transient boolean referenceVisualizationEnabled = false;
+	@Getter
+	private transient final SelectionModel selectionModel;
 
 	public MappingModel() {
 		this.relations = new HashSet<>();
 		this.changeListeners = new HashSet<>();
+		this.selectionModel = new SelectionModel(this);
 	}
 
 	public void addRelation(Relation r) {
@@ -51,14 +52,6 @@ public class MappingModel implements Viewable {
 		}
 	}
 
-	/**
-	 * Gets the list of relations which are currently selected.
-	 * @return The list of relations which are selected.
-	 */
-	public List<Relation> getSelectedRelations() {
-		return this.relations.stream().filter(Relation::isSelected).collect(Collectors.toList());
-	}
-
 	/**
 	 * Finds an attribute in this model, or returns null otherwise.
 	 * @param relationName The name of the relation the attribute is in.
@@ -86,11 +79,8 @@ public class MappingModel implements Viewable {
 		for (Relation r : this.getRelations()) {
 			Set<Attribute> removalSet = new HashSet<>();
 			for (Attribute a : r.getAttributes()) {
-				if (a instanceof ForeignKeyAttribute) {
+				if (a.hasReference() && a.getReference().equals(referenced)) {
-					ForeignKeyAttribute fkA = (ForeignKeyAttribute) a;
+					removalSet.add(a);
-					if (fkA.getReference().equals(referenced)) {
-						removalSet.add(fkA);
-					}
 				}
 			}
 			removalSet.forEach(r::removeAttribute);
@@ -196,12 +186,11 @@ public class MappingModel implements Viewable {
 				ObjectNode attributeNode = mapper.createObjectNode()
 						.put("name", a.getName())
 						.put("type", a.getType().name());
-				if (a instanceof ForeignKeyAttribute) {
+				if (a.hasReference()) {
-					ForeignKeyAttribute fk = (ForeignKeyAttribute) a;
 					ObjectNode referenceNode = mapper.createObjectNode()
-							.put("relation", fk.getReference().getRelation().getName())
+							.put("relation", a.getReference().getRelation().getName())
-							.put("attribute", fk.getReference().getName());
+							.put("attribute", a.getReference().getName());
-					attributeNode.set("references", referenceNode);
+					attributeNode.set("reference", referenceNode);
 				}
 				attributesArray.add(attributeNode);
 			}
@@ -245,7 +234,8 @@ public class MappingModel implements Viewable {
 				Attribute referencedAttribute = model.findAttribute(referencedRelation, referencedName);
 				if (referencedAttribute == null) throw new IllegalArgumentException("Foreign key referenced unknown attribute.");
 				if (!references.containsKey(referencedAttribute)) {
-					ForeignKeyAttribute fk = new ForeignKeyAttribute(attribute.getRelation(), attribute.getType(), attribute.getName(), referencedAttribute);
+					Attribute fk = new Attribute(attribute.getRelation(), attribute.getType(), attribute.getName());
+					fk.setReference(referencedAttribute);
 					attribute.getRelation().removeAttribute(attribute);
 					attribute.getRelation().addAttribute(fk);
 					references.remove(attribute);
@@ -270,8 +260,8 @@ public class MappingModel implements Viewable {
 		Map<Attribute, ObjectNode> references = new HashMap<>();
 		for (JsonNode r : node.withArray("relations")) {
 			for (JsonNode a : r.withArray("attributes")) {
-				if (a.has("references") && a.get("references").isObject()) {
+				if (a.has("reference") && a.get("reference").isObject()) {
-					ObjectNode referenceNode = (ObjectNode) a.get("references");
+					ObjectNode referenceNode = (ObjectNode) a.get("reference");
 					String attributeName = a.get("name").asText();
 					String relationName = r.get("name").asText();
 					Attribute attribute = model.findAttribute(relationName, attributeName);

src/main/java/nl/andrewlalis/erme/model/Relation.java

@@ -20,7 +20,6 @@ public class Relation implements Viewable, Comparable<Relation> {
 	private String name;
 	private final List<Attribute> attributes;
 
-	private transient boolean selected;
 	private transient RelationViewModel viewModel;
 
 	public Relation(MappingModel model, Point position, String name, List<Attribute> attributes) {
@@ -42,10 +41,6 @@ public class Relation implements Viewable, Comparable<Relation> {
 		this.name = name;
 	}
 
-	public void setSelected(boolean selected) {
-		this.selected = selected;
-	}
-
 	public void addAttribute(Attribute attribute) {
 		this.attributes.add(attribute);
 		this.model.fireChangedEvent();
@@ -63,6 +58,10 @@ public class Relation implements Viewable, Comparable<Relation> {
 		}
 	}
 
+	public boolean isSelected() {
+		return this.model.getSelectionModel().isSelected(this);
+	}
+
 	@Override
 	public ViewModel getViewModel() {
 		if (this.viewModel == null) {

src/main/java/nl/andrewlalis/erme/model/SelectionModel.java

@@ -0,0 +1,75 @@
+package nl.andrewlalis.erme.model;
+
+import java.util.*;
+
+/**
+ * A model that keeps track of the application's set of selected elements.
+ */
+public class SelectionModel {
+	private final MappingModel model;
+	private final Set<Relation> selectedRelations;
+	private final Set<Attribute> selectedAttributes;
+
+	public SelectionModel(MappingModel model) {
+		this.model = model;
+		this.selectedRelations = new HashSet<>();
+		this.selectedAttributes = new HashSet<>();
+	}
+
+	public boolean isSelected(Relation relation) {
+		return this.selectedRelations.contains(relation);
+	}
+
+	public boolean isSelected(Attribute attribute) {
+		return this.selectedAttributes.contains(attribute);
+	}
+
+	public List<Relation> getSelectedRelations() {
+		return new ArrayList<>(this.selectedRelations);
+	}
+
+	public List<Attribute> getSelectedAttributes() {
+		return new ArrayList<>(this.selectedAttributes);
+	}
+
+	public void select(Relation relation) {
+		this.selectedRelations.add(relation);
+	}
+
+	public void selectAll(Collection<Relation> relations) {
+		this.selectedRelations.addAll(relations);
+	}
+
+	public void select(Attribute attribute) {
+		this.selectedAttributes.add(attribute);
+	}
+
+	public void toggle(Relation relation) {
+		if (selectedRelations.contains(relation)) {
+			selectedRelations.remove(relation);
+		} else {
+			selectedRelations.add(relation);
+		}
+	}
+
+	public void toggle(Attribute attribute) {
+		if (selectedAttributes.contains(attribute)) {
+			selectedAttributes.remove(attribute);
+		} else {
+			selectedAttributes.add(attribute);
+		}
+	}
+
+	public void clearSelection() {
+		this.selectedRelations.clear();
+		this.selectedAttributes.clear();
+	}
+
+	public void clearRelations() {
+		this.selectedRelations.clear();
+	}
+
+	public void clearAttributes() {
+		this.selectedAttributes.clear();
+	}
+}

src/main/java/nl/andrewlalis/erme/view/DiagramPopupMenu.java

@@ -2,7 +2,9 @@ package nl.andrewlalis.erme.view;
 
 import nl.andrewlalis.erme.control.actions.edits.AddAttributeAction;
 import nl.andrewlalis.erme.control.actions.edits.AddRelationAction;
+import nl.andrewlalis.erme.control.actions.edits.RemoveAttributeAction;
 import nl.andrewlalis.erme.control.actions.edits.RemoveRelationAction;
+import nl.andrewlalis.erme.model.Attribute;
 import nl.andrewlalis.erme.model.Relation;
 
 import javax.swing.*;
@@ -10,19 +12,19 @@ import java.util.List;
 
 public class DiagramPopupMenu extends JPopupMenu {
 	public DiagramPopupMenu(DiagramPanel diagramPanel) {
-		List<Relation> selectedRelations = diagramPanel.getModel().getSelectedRelations();
+		List<Relation> selectedRelations = diagramPanel.getModel().getSelectionModel().getSelectedRelations();
-		if (selectedRelations.size() == 0) {
+		List<Attribute> selectedAttributes = diagramPanel.getModel().getSelectionModel().getSelectedAttributes();
+		if (selectedRelations.isEmpty() && selectedAttributes.isEmpty()) {
 			this.add(new AddRelationAction(diagramPanel));
 		}
-		if (selectedRelations.size() > 0) {
+		if (selectedRelations.size() > 0 && selectedAttributes.isEmpty()) {
 			this.add(new RemoveRelationAction(diagramPanel));
 		}
-		if (selectedRelations.size() == 1) {
+		if (selectedRelations.size() == 1 && selectedAttributes.isEmpty()) {
-			Relation relation = selectedRelations.get(0);
 			this.add(new AddAttributeAction(diagramPanel));
-			if (!relation.getAttributes().isEmpty()) {
+		}
-				this.add(new RemoveRelationAction(diagramPanel));
+		if (!selectedAttributes.isEmpty() && selectedRelations.isEmpty()) {
-			}
+			this.add(new RemoveAttributeAction(diagramPanel));
 		}
 	}
 }

src/main/java/nl/andrewlalis/erme/view/EditAttributePopupDialog.java

@@ -0,0 +1,96 @@
+package nl.andrewlalis.erme.view;
+
+import nl.andrewlalis.erme.model.Attribute;
+import nl.andrewlalis.erme.model.AttributeType;
+import nl.andrewlalis.erme.model.Relation;
+
+import javax.swing.*;
+import java.awt.*;
+
+/**
+ * A popup that's shown when creating or editing an attribute of a relation.
+ */
+public class EditAttributePopupDialog extends JDialog {
+	private final Relation relation;
+	private final Attribute attribute;
+
+	private final JTextField nameField;
+	private final JComboBox<AttributeType> attributeTypeComboBox;
+	private final JSpinner orderSpinner;
+
+	public EditAttributePopupDialog(Frame owner, Relation relation, Attribute attribute) {
+		super(owner, "Edit Attribute", true);
+		this.relation = relation;
+		this.attribute = attribute;
+		JPanel mainPanel = new JPanel(new GridBagLayout());
+		GridBagConstraints gc = new GridBagConstraints();
+		gc.ipadx = 2;
+		gc.ipady = 2;
+		gc.gridx = 0;
+		gc.gridy = 0;
+		gc.anchor = GridBagConstraints.LINE_START;
+		mainPanel.add(new JLabel("Name"), gc);
+		gc.gridx = 1;
+		gc.anchor = GridBagConstraints.LINE_END;
+		this.nameField = new JTextField(20);
+		mainPanel.add(nameField, gc);
+		gc.gridx = 0;
+		gc.gridy++;
+		gc.anchor = GridBagConstraints.LINE_START;
+		mainPanel.add(new JLabel("Type"), gc);
+		gc.gridx = 1;
+		gc.anchor = GridBagConstraints.LINE_END;
+		this.attributeTypeComboBox = new JComboBox<>(AttributeType.values());
+		mainPanel.add(attributeTypeComboBox, gc);
+		gc.gridx = 0;
+		gc.gridy++;
+		gc.anchor = GridBagConstraints.LINE_START;
+		mainPanel.add(new JLabel("Order"), gc);
+		gc.gridx = 1;
+		gc.anchor = GridBagConstraints.LINE_END;
+		this.orderSpinner = new JSpinner(new SpinnerNumberModel(1, 1, relation.getAttributes().size() + 1, 1));
+		mainPanel.add(orderSpinner, gc);
+
+		JPanel buttonPanel = new JPanel(new FlowLayout(FlowLayout.RIGHT));
+		JButton okayButton = new JButton("Okay");
+		okayButton.addActionListener(e -> this.submit());
+		JButton cancelButton = new JButton("Cancel");
+		cancelButton.addActionListener(e -> this.dispose());
+		buttonPanel.add(okayButton);
+		buttonPanel.add(cancelButton);
+		gc.gridx = 0;
+		gc.gridy++;
+		gc.gridwidth = 2;
+		mainPanel.add(buttonPanel, gc);
+
+		if (this.attribute != null) {
+			this.nameField.setText(this.attribute.getName());
+			this.attributeTypeComboBox.setSelectedItem(this.attribute.getType());
+			this.orderSpinner.setValue(this.relation.getAttributes().indexOf(this.attribute));
+		} else {
+			this.orderSpinner.setValue(this.relation.getAttributes().size() + 1);
+		}
+
+		this.setContentPane(mainPanel);
+		this.pack();
+		this.setLocationRelativeTo(owner);
+	}
+
+	private void submit() {
+		String name = this.nameField.getText().trim();
+		AttributeType type = (AttributeType) this.attributeTypeComboBox.getSelectedItem();
+		int order = (int) this.orderSpinner.getValue();
+		if (name.isEmpty()) {
+			JOptionPane.showMessageDialog(this, "The attribute must have a name.", "Missing Name", JOptionPane.WARNING_MESSAGE);
+			return;
+		}
+		if (this.attribute != null) {
+			this.attribute.setName(name);
+			this.attribute.setType(type);
+		} else {
+			Attribute a = new Attribute(this.relation, type, name);
+			this.relation.addAttribute(a, order - 1);
+		}
+		this.dispose();
+	}
+}

src/main/java/nl/andrewlalis/erme/view/EditorMenuBar.java

@@ -3,6 +3,7 @@ package nl.andrewlalis.erme.view;
 import nl.andrewlalis.erme.control.actions.*;
 import nl.andrewlalis.erme.control.actions.edits.AddAttributeAction;
 import nl.andrewlalis.erme.control.actions.edits.AddRelationAction;
+import nl.andrewlalis.erme.control.actions.edits.RemoveAttributeAction;
 import nl.andrewlalis.erme.control.actions.edits.RemoveRelationAction;
 
 import javax.swing.*;
@@ -35,7 +36,7 @@ public class EditorMenuBar extends JMenuBar {
 		menu.add(new AddRelationAction(diagramPanel));
 		menu.add(new RemoveRelationAction(diagramPanel));
 		menu.add(new AddAttributeAction(diagramPanel));
-		menu.add(new RemoveRelationAction(diagramPanel));
+		menu.add(new RemoveAttributeAction(diagramPanel));
 		menu.add(new AutoPositionAction(diagramPanel));
 		return menu;
 	}

src/main/java/nl/andrewlalis/erme/view/view_models/AttributeViewModel.java

@@ -2,7 +2,6 @@ package nl.andrewlalis.erme.view.view_models;
 
 import nl.andrewlalis.erme.model.Attribute;
 import nl.andrewlalis.erme.model.AttributeType;
-import nl.andrewlalis.erme.model.ForeignKeyAttribute;
 
 import java.awt.*;
 import java.awt.font.TextAttribute;
@@ -16,6 +15,7 @@ public class AttributeViewModel implements ViewModel {
 	public static final int PADDING_X = 5;
 	public static final int PADDING_Y = 5;
 	public static final Color BACKGROUND_COLOR = new Color(192, 192, 192, 127);
+	public static final Color SELECTED_COLOR = new Color(148, 255, 176, 127);
 	public static final Color FONT_COLOR = Color.BLACK;
 	public static final float FK_FONT_SIZE = 11.0f;
 	private static final float LOLCAT_SAT = 0.75f;
@@ -35,16 +35,16 @@ public class AttributeViewModel implements ViewModel {
 		g.setColor(FONT_COLOR);
 		g.drawRect(r.x, r.y, r.width, r.height);
 		g.drawString(as.getIterator(), r.x + PADDING_X, r.y + (r.height - PADDING_Y));
-		if (this.attribute instanceof ForeignKeyAttribute) {
+		if (this.attribute.hasReference() && !this.attribute.getRelation().getModel().isReferenceVisualizationEnabled()) {
-			ForeignKeyAttribute fkAttribute = (ForeignKeyAttribute) this.attribute;
 			Font originalFont = g.getFont();
 			g.setFont(g.getFont().deriveFont(Font.ITALIC, FK_FONT_SIZE));
-			g.drawString(fkAttribute.getFullReferenceName(), r.x + PADDING_X, r.y - PADDING_Y);
+			g.drawString(getFullReferenceName(), r.x + PADDING_X, r.y - PADDING_Y);
 			g.setFont(originalFont);
 		}
 	}
 
 	private Color getBackgroundColor(int x, int y, Graphics2D g) {
+		if (attribute.isSelected()) return SELECTED_COLOR;
 		if (!attribute.getRelation().getModel().isLolcatEnabled()) return BACKGROUND_COLOR;
 		Point offset = g.getClipBounds().getLocation();
 		g.translate(offset.x, offset.y);
@@ -82,11 +82,10 @@ public class AttributeViewModel implements ViewModel {
 		Rectangle2D nameRect = g.getFontMetrics().getStringBounds(as.getIterator(), 0, this.attribute.getName().length(), g);
 		int width = (int) nameRect.getWidth() + (2 * PADDING_X);
 		int height = (int) nameRect.getHeight() + (2 * PADDING_Y);
-		if (this.attribute instanceof ForeignKeyAttribute) {
+		if (this.attribute.hasReference()) {
-			ForeignKeyAttribute fkAttribute = (ForeignKeyAttribute) this.attribute;
 			Font originalFont = g.getFont();
 			g.setFont(g.getFont().deriveFont(Font.ITALIC, FK_FONT_SIZE));
-			Rectangle referenceNameBounds = g.getFontMetrics().getStringBounds(fkAttribute.getFullReferenceName(), g).getBounds();
+			Rectangle referenceNameBounds = g.getFontMetrics().getStringBounds(getFullReferenceName(), g).getBounds();
 			g.setFont(originalFont);
 			width = Math.max(width, referenceNameBounds.width + (2 * PADDING_X));
 		}
@@ -103,4 +102,9 @@ public class AttributeViewModel implements ViewModel {
 		}
 		return as;
 	}
+
+	private String getFullReferenceName() {
+		if (!this.attribute.hasReference()) return "";
+		return this.attribute.getReference().getRelation().getName() + "." + this.attribute.getReference().getName();
+	}
 }

src/main/java/nl/andrewlalis/erme/view/view_models/MappingModelViewModel.java

@@ -1,7 +1,6 @@
 package nl.andrewlalis.erme.view.view_models;
 
 import nl.andrewlalis.erme.model.Attribute;
-import nl.andrewlalis.erme.model.ForeignKeyAttribute;
 import nl.andrewlalis.erme.model.MappingModel;
 import nl.andrewlalis.erme.model.Relation;
 
@@ -32,13 +31,12 @@ public class MappingModelViewModel implements ViewModel {
 		g2.setStroke(dashedStroke);
 		for (Relation r : this.model.getRelations()) {
 			for (Attribute a : r.getAttributes()) {
-				if (a instanceof ForeignKeyAttribute) {
+				if (a.hasReference()) {
-					ForeignKeyAttribute fk = (ForeignKeyAttribute) a;
 					// Generate a random HSB color for the line, seeded using the referenced attribute's hash code.
-					Random random = new Random(fk.getReference().hashCode());
+					Random random = new Random(a.getReference().hashCode());
 					g2.setColor(Color.getHSBColor(random.nextFloat(), 1.0f, 0.8f));
-					Rectangle sourceBounds = fk.getViewModel().getBounds(g);
+					Rectangle sourceBounds = a.getViewModel().getBounds(g);
-					Rectangle targetBounds = fk.getReference().getViewModel().getBounds(g);
+					Rectangle targetBounds = a.getReference().getViewModel().getBounds(g);
 					Point sourcePoint = new Point(sourceBounds.x + sourceBounds.width / 2, sourceBounds.y + 3 * targetBounds.height / 4);
 					Point targetPoint = new Point(targetBounds.x + targetBounds.width / 2, targetBounds.y + 3 * targetBounds.height / 4);
 					g2.drawLine(sourcePoint.x, sourcePoint.y, targetPoint.x, targetPoint.y);