ToMany: Set an comparator to define the order of entities

Author: greenrobot

objectbox-java/src/main/java/io/objectbox/relation/ToMany.java

@@ -72,6 +72,7 @@ public class ToMany<TARGET> implements List<TARGET>, Serializable {
     transient private Box entityBox;
     transient private volatile Box<TARGET> targetBox;
     transient private boolean removeFromTargetBox;
+    transient private Comparator<TARGET> comparator;
 
     public ToMany(Object sourceEntity, RelationInfo<TARGET> relationInfo) {
         if(sourceEntity == null ) {
@@ -93,6 +94,12 @@ public void setListFactory(ListFactory listFactory) {
         this.listFactory = listFactory;
     }
 
+    /** Set an comparator to define the order of entities. */
+    @Experimental
+    public void setComparator(Comparator<TARGET> comparator) {
+        this.comparator = comparator;
+    }
+
     /**
      * On put, this also deletes removed entities from the target Box.
      * Note: removed target entities won't cascade the delete.
@@ -162,6 +169,9 @@ private void ensureEntities() {
                     newEntities = targetBox.internalGetBacklinkEntities(relationInfo.targetInfo.getEntityId(),
                             relationInfo.targetIdProperty, id);
                 }
+                if(comparator != null) {
+                    Collections.sort(newEntities, comparator);
+                }
                 synchronized (this) {
                     if (entities == null) {
                         entities = newEntities;

tests/objectbox-java-test/src/main/java/io/objectbox/relation/RelationTest.java

@@ -18,6 +18,7 @@
 
 import org.junit.Test;
 
+import java.util.Comparator;
 import java.util.List;
 
 import io.objectbox.query.Query;
@@ -55,6 +56,28 @@ public void testRelationToMany() {
         assertEquals("Oranges", orders.get(1).getText());
     }
 
+    @Test
+    public void testRelationToMany_comparator() {
+        Customer customer = putCustomer();
+        putOrder(customer, "Bananas");
+        putOrder(customer, "Oranges");
+        putOrder(customer, "Apples");
+
+        ToMany<Order> orders = (ToMany<Order>) customer.getOrders();
+        orders.setComparator(new Comparator<Order>() {
+            @Override
+            public int compare(Order o1, Order o2) {
+                return o1.text.compareTo(o2.text);
+            }
+        });
+        orders.reset();
+
+        assertEquals(3, orders.size());
+        assertEquals("Apples", orders.get(0).getText());
+        assertEquals("Bananas", orders.get(1).getText());
+        assertEquals("Oranges", orders.get(2).getText());
+    }
+
     @Test
     public void testRelationToMany_activeRelationshipChanges() {
         Customer customer = putCustomer();