Merge union type into schem builder when using Merge (#477)

Author: pekkah

src/graphql/SDL/SdlReader.cs

@@ -201,7 +201,7 @@ protected IType OutputType(GraphQLType typeDefinition)
 
             if (definition == null)
                 throw new InvalidOperationException(
-                    $"Could not find a input type definition '{typeName}'.");
+                    $"Could not find a output type definition '{typeName}'.");
 
             switch (definition)
             {

src/graphql/Tools/MergeTool.cs

@@ -38,11 +38,21 @@ public static void Schema(SchemaBuilder target, ISchema right)
             foreach (var rightType in right.QueryTypes<ComplexType>())
                 MergeComplexType(right, target, rightType);
 
+            // merge unions
+            foreach (var rightType in right.QueryTypes<UnionType>())
+                MergeUnionType(right, target, rightType);
+
             // merge complex type field
             foreach (var rightType in right.QueryTypes<ComplexType>())
                 MergeComplexTypeFields(right, target, rightType);
         }
 
+        private static void MergeUnionType(ISchema right, SchemaBuilder builder, UnionType rightType)
+        {
+            if (!builder.TryGetType<UnionType>(rightType.Name, out _)) 
+                builder.Include(rightType);
+        }
+
         private static void MergeEnumType(ISchema right, SchemaBuilder builder, EnumType rightType)
         {
             // merge values if enum exists in both

tests/graphql.tests/Tools/MergeSchemasFacts.cs

@@ -121,5 +121,47 @@ type Query {
             var field = schema.GetField("Query", "currentColor");
             Assert.Equal(newEnumType, field.Type);
         }
+
+        [Fact]
+        public void Merge_schema_with_new_union()
+        {
+            /* Given */
+            var newTypes = new SchemaBuilder()
+                .Sdl(@"
+type Red {
+}
+
+type Green {
+}
+
+type Orange {
+}
+
+union Color = Red | Orange | Green
+
+type Query {
+    color: Color
+}
+                    ")
+                .Build();
+
+            var builder = new SchemaBuilder()
+                .Sdl(@"
+                    type Query {
+                    }
+                    ");
+
+            
+            /* When */
+            var schema = builder.Merge(newTypes)
+                .Build();
+
+            /* Then */
+            var newUnionType = schema.GetNamedType<UnionType>("Color");
+            Assert.NotNull(newUnionType);
+
+            var field = schema.GetField("Query", "color");
+            Assert.Equal(newUnionType, field.Type);
+        }
     }
 }