Add sanity checks for schema to allow only a single query, mutation, subscription in schema

Related GraphQL-js commit: graphql/graphql-js@ffe76c5

Author: syrusakbary

graphql/utils/build_ast_schema.py

@@ -82,10 +82,16 @@ def build_ast_schema(document):
     for operation_type in schema_def.operation_types:
         type_name = operation_type.type.name.value
         if operation_type.operation == 'query':
+            if query_type_name:
+                raise Exception('Must provide only one query type in schema.')
             query_type_name = type_name
         elif operation_type.operation == 'mutation':
+            if mutation_type_name:
+                raise Exception('Must provide only one mutation type in schema.')
             mutation_type_name = type_name
         elif operation_type.operation == 'subscription':
+            if subscription_type_name:
+                raise Exception('Must provide only one subscription type in schema.')
             subscription_type_name = type_name
 
     if not query_type_name:

graphql/utils/tests/test_build_ast_schema.py

@@ -430,6 +430,74 @@ def test_requires_a_query_type():
     assert 'Must provide schema definition with query type.' == str(excinfo.value)
 
 
+def test_allows_only_a_single_query_type():
+    body = '''
+schema {
+  query: Hello
+  query: Yellow
+}
+
+type Hello {
+  bar: Bar
+}
+
+type Yellow {
+  isColor: Boolean
+}
+'''
+    doc = parse(body)
+    with raises(Exception) as excinfo:
+        build_ast_schema(doc)
+
+    assert 'Must provide only one query type in schema.' == str(excinfo.value)
+
+
+def test_allows_only_a_single_mutation_type():
+    body = '''
+schema {
+  query: Hello
+  mutation: Hello
+  mutation: Yellow
+}
+
+type Hello {
+  bar: Bar
+}
+
+type Yellow {
+  isColor: Boolean
+}
+'''
+    doc = parse(body)
+    with raises(Exception) as excinfo:
+        build_ast_schema(doc)
+
+    assert 'Must provide only one mutation type in schema.' == str(excinfo.value)
+
+
+def test_allows_only_a_single_subscription_type():
+    body = '''
+schema {
+  query: Hello
+  subscription: Hello
+  subscription: Yellow
+}
+
+type Hello {
+  bar: Bar
+}
+
+type Yellow {
+  isColor: Boolean
+}
+'''
+    doc = parse(body)
+    with raises(Exception) as excinfo:
+        build_ast_schema(doc)
+
+    assert 'Must provide only one subscription type in schema.' == str(excinfo.value)
+
+
 def test_unknown_type_referenced():
     body = '''
 schema {