diff --git a/teaql-core/src/test/java/io/teaql/core/EntityKeyTest.java b/teaql-core/src/test/java/io/teaql/core/EntityKeyTest.java
new file mode 100644
index 0000000..64c056e
--- /dev/null
+++ b/teaql-core/src/test/java/io/teaql/core/EntityKeyTest.java
@@ -0,0 +1,72 @@
+package io.teaql.core;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotEquals;
+import static org.junit.Assert.assertThrows;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.TreeSet;
+import org.junit.Test;
+
+public class EntityKeyTest {
+
+    @Test
+    public void equalKeysHaveEqualHashCodes() {
+        EntityKey first = new EntityKey("Order", 42L);
+        EntityKey second = new EntityKey("Order", 42L);
+
+        assertEquals(first, second);
+        assertEquals(first.hashCode(), second.hashCode());
+    }
+
+    @Test
+    public void entityTypeAndIdBothParticipateInEquality() {
+        EntityKey key = new EntityKey("Order", 42L);
+
+        assertNotEquals(key, new EntityKey("OrderItem", 42L));
+        assertNotEquals(key, new EntityKey("Order", 43L));
+        assertNotEquals(key, null);
+        assertNotEquals(key, "Order:42");
+    }
+
+    @Test
+    public void naturalOrderingSortsByEntityTypeThenId() {
+        TreeSet<EntityKey> keys = new TreeSet<>();
+        keys.add(new EntityKey("OrderItem", 1L));
+        keys.add(new EntityKey("Order", 2L));
+        keys.add(new EntityKey("Order", 1L));
+
+        assertEquals(
+                List.of(
+                        new EntityKey("Order", 1L),
+                        new EntityKey("Order", 2L),
+                        new EntityKey("OrderItem", 1L)),
+                new ArrayList<>(keys));
+    }
+
+    @Test
+    public void nullIdSortsBeforeNonNullIdOfSameEntityType() {
+        TreeSet<EntityKey> keys = new TreeSet<>();
+        keys.add(new EntityKey("Order", 1L));
+        keys.add(new EntityKey("Order", null));
+
+        assertEquals(
+                List.of(new EntityKey("Order", null), new EntityKey("Order", 1L)),
+                new ArrayList<>(keys));
+    }
+
+    @Test
+    public void nullEntityTypeIsRejected() {
+        NullPointerException error =
+                assertThrows(NullPointerException.class, () -> new EntityKey(null, 1L));
+
+        assertEquals("entity type must not be null", error.getMessage());
+    }
+
+    @Test
+    public void stringRepresentationUsesEntityAndId() {
+        assertEquals("Order:42", new EntityKey("Order", 42L).toString());
+        assertEquals("Order:null", new EntityKey("Order", null).toString());
+    }
+}