Extend ThingConstraint.has to return attribute identity variables

Author: krishnangovindraj

java/pattern/constraint/ThingConstraint.java

@@ -41,6 +41,7 @@
 import java.util.Set;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.regex.Pattern;
+import java.util.stream.Collectors;
 
 import static com.vaticle.typedb.common.collection.Collections.list;
 import static com.vaticle.typedb.common.collection.Collections.pair;
@@ -436,7 +437,7 @@ private Has(@Nullable TypeVariable type, ThingVariable<?> attribute) {
 
         @Override
         public Set<BoundVariable> variables() {
-            return set(attribute);
+            return attribute.variables().collect(Collectors.toSet());
         }
 
         @Override

java/query/test/TypeQLQueryTest.java

@@ -142,6 +142,14 @@ public void testMatchInsertWithValueVariable() {
         assertEquals("match\n?x = 2;\ninsert\n$a ?x isa prime;", query.toString());
     }
 
+    @Test
+    public void testMatchInsertOwnershipWithValueVariable() {
+        TypeQLInsert query = match(
+                valvar("x").assign(Expr.constant(2))
+        ).insert(var("p").has("prime", valvar("x")));
+        assertEquals("match\n?x = 2;\ninsert\n$p has prime ?x;", query.toString());
+    }
+
     @Test
     public void testZeroToString() {
         assertEquals("match\n$x 0.0;", match(var("x").eq(0.0)).toString());