Prevent registering more than one types on the same name

src/type/__tests__/schema.js

@@ -0,0 +1,42 @@
+/**
+ *  Copyright (c) 2015, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the BSD-style license found in the
+ *  LICENSE file in the root directory of this source tree. An additional grant
+ *  of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+import {
+  GraphQLSchema,
+  GraphQLObjectType,
+} from '../';
+
+import { describe, it } from 'mocha';
+import { expect } from 'chai';
+
+describe('Type System: Schema', () => {
+  it('does not allow more than one type of the same name', () => {
+    var A = new GraphQLObjectType({
+      name: 'SameName',
+      fields: {}
+    });
+
+    var B = new GraphQLObjectType({
+      name: 'SameName',
+      fields: {}
+    });
+
+    var SomeQuery = new GraphQLObjectType({
+      name: 'SomeQuery',
+      fields: {
+        a: { type: A },
+        b: { type: B }
+      }
+    });
+
+    expect(
+      () => new GraphQLSchema({ query: SomeQuery })
+    ).to.throw('Schema cannot contain more than one type named SameName.');
+  });
+});

src/type/schema.js

@@ -44,6 +44,7 @@ export class GraphQLSchema {
 
   constructor(config: GraphQLSchemaConfig) {
     this._schemaConfig = config;
+    this._typeMap = buildTypeMap(this);
   }
 
   getQueryType(): GraphQLObjectType {
@@ -55,11 +56,7 @@ export class GraphQLSchema {
   }
 
   getTypeMap(): TypeMap {
-    return this._typeMap || (this._typeMap = [
-      this.getQueryType(),
-      this.getMutationType(),
-      __Schema
-    ].reduce(typeMapReducer, {}));
+    return this._typeMap;
   }
 
   getType(name: string): ?GraphQLType {
@@ -85,11 +82,28 @@ type GraphQLSchemaConfig = {
   mutation?: ?GraphQLObjectType;
 }
 
+function buildTypeMap(schema: GraphQLSchema): TypeMap {
+  return [
+    schema.getQueryType(),
+    schema.getMutationType(),
+    __Schema
+  ].reduce(typeMapReducer, {});
+}
+
 export function typeMapReducer(map: TypeMap, type: ?GraphQLType): TypeMap {
   if (type instanceof GraphQLList || type instanceof GraphQLNonNull) {
     return typeMapReducer(map, type.ofType);
   }
-  if (!type || map[type.name]) {
+  if (!type) {
+    return map;
+  }
+  var prevType = map[type.name];
+  if (prevType) {
+    if (prevType !== type) {
+      throw new Error(
+        `Schema cannot contain more than one type named ${type.name}.`
+      );
+    }
     return map;
   }
   map[type.name] = type;