Discriminator added to all Queries when Polymorphism is not enabled

core/src/main/java/dev/morphia/mapping/Mapper.java

@@ -541,14 +541,22 @@ public void updateQueryWithDiscriminators(EntityModel model, Document query) {
                 && !query.containsKey("_id")
                 && !query.containsKey(model.getDiscriminatorKey())) {
             List<String> values = new ArrayList<>();
-            values.add(model.getDiscriminator());
+            List<EntityModel> classesMappedToCollection = getClassesMappedToCollection(model.getCollectionName());
+            // the discriminator is added here if size > 1 because, unless polymorphic queries are enabled, historically Morphia has only
+            // returned the type given to the query. adding the discriminator value here ensures that only the query type is returned
+            // unless polymorphic queries are explicitly enabled.
+            if (classesMappedToCollection.size() > 1 || config.enablePolymorphicQueries()) {
+                values.add(model.getDiscriminator());
+            }
             if (config.enablePolymorphicQueries()) {
                 for (EntityModel subtype : model.getSubtypes()) {
                     values.add(subtype.getDiscriminator());
                 }
             }
-            query.put(model.getDiscriminatorKey(),
-                    new Document("$in", values));
+            if (!values.isEmpty()) {
+                query.put(model.getDiscriminatorKey(),
+                        new Document("$in", values));
+            }
         }
     }
 

core/src/main/java/dev/morphia/query/LegacyQuery.java

@@ -1,6 +1,5 @@
 package dev.morphia.query;
 
-import java.util.ArrayList;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
@@ -24,6 +23,7 @@
 import dev.morphia.annotations.Entity;
 import dev.morphia.annotations.internal.MorphiaInternal;
 import dev.morphia.internal.MorphiaInternals.DriverVersion;
+import dev.morphia.mapping.Mapper;
 import dev.morphia.mapping.codec.pojo.EntityModel;
 import dev.morphia.query.internal.MorphiaCursor;
 import dev.morphia.query.internal.MorphiaKeyCursor;
@@ -435,19 +435,16 @@ public Document getSort() {
     @MorphiaInternal
     public Document toDocument() {
         final Document query = getQueryDocument();
-        EntityModel model = datastore.getMapper().getEntityModel(getEntityClass());
+        Mapper mapper = datastore.getMapper();
+        EntityModel model = mapper.getEntityModel(getEntityClass());
         Entity entityAnnotation = model.getEntityAnnotation();
         if (entityAnnotation != null && entityAnnotation.useDiscriminator()
                 && !query.containsKey("_id")
                 && !query.containsKey(model.getDiscriminatorKey())) {
 
-            List<String> values = new ArrayList<>();
-            values.add(model.getDiscriminator());
-            for (EntityModel subtype : datastore.getMapper().getEntityModel(getEntityClass()).getSubtypes()) {
-                values.add(subtype.getDiscriminator());
+            if (mapper.isMappable(getEntityClass())) {
+                mapper.updateQueryWithDiscriminators(mapper.getEntityModel(getEntityClass()), query);
             }
-            query.put(model.getDiscriminatorKey(),
-                    new Document("$in", values));
         }
         return query;
     }

core/src/test/java/dev/morphia/test/mapping/codec/TestDocumentWriter.java

@@ -52,8 +52,6 @@ public void testOr() {
                 of(
                         new MergingDocument("$or", of(new Document("name", "A"), new Document("name", "B"))),
                         new MergingDocument("$or", of(new Document("name", "C"), new Document("name", "D")))));
-        System.out.println("document = " + document.toJson(JSON_WRITER_SETTINGS));
-        System.out.println("expected = " + expected.toJson(JSON_WRITER_SETTINGS));
         assertDocumentEquals(document, expected);
     }
 

core/src/test/java/dev/morphia/test/models/TestDiscriminatorEntity.java

@@ -0,0 +1,34 @@
+/*
+  Copyright (C) 2010 Olafur Gauti Gudmundsson
+  <p/>
+  Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may
+  obtain a copy of the License at
+  <p/>
+  http://www.apache.org/licenses/LICENSE-2.0
+  <p/>
+  Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions
+  and limitations under the License.
+ */
+
+package dev.morphia.test.models;
+
+import dev.morphia.annotations.Entity;
+import dev.morphia.annotations.Id;
+
+import org.bson.types.ObjectId;
+
+@Entity
+public class TestDiscriminatorEntity {
+    @Id
+    public ObjectId id;
+    public String something;
+
+    public ObjectId getId() {
+        return id;
+    }
+
+    public void setId(ObjectId id) {
+        this.id = id;
+    }
+}

core/src/test/java/dev/morphia/test/models/TestEntity.java

@@ -22,6 +22,7 @@
 public class TestEntity {
     @Id
     public ObjectId id;
+    public String something;
 
     public ObjectId getId() {
         return id;