docs(interfaces-unions): add info about support for resolveType

docs/interfaces.md

@@ -6,7 +6,9 @@ The main idea of TypeGraphQL is to create GraphQL types based on TypeScript clas
 
 In object-oriented programming it is common to create interfaces which describe the contract that classes implementing them must adhere to. Hence, TypeGraphQL supports defining GraphQL interfaces.
 
-## How to
+Read more about the GraphQL Interface Type in the [official GraphQL docs](https://graphql.org/learn/schema/#interfaces).
+
+## Usage
 
 TypeScript has first class support for interfaces. Unfortunately, they only exist at compile-time, so we can't use them to build GraphQL schema at runtime by using decorators.
 
@@ -43,8 +45,28 @@ The only difference is that we have to let TypeGraphQL know that this `ObjectTyp
 
 We can also omit the decorators since the GraphQL types will be copied from the interface definition - this way we won't have to maintain two definitions and solely rely on TypeScript type checking for correct interface implementation.
 
+## Resolving Type
+
 Be aware that when our object type is implementing a GraphQL interface type, **we have to return an instance of the type class** in our resolvers. Otherwise, `graphql-js` will not be able to detect the underlying GraphQL type correctly.
 
+We can also provide our own `resolveType` function implementation to the `@InterfaceType` options. This way we can return plain objects in resolvers and then determine the returned object type by checking the shape of the data object, the same ways [like in unions](./unions.md), e.g.:
+
+```typescript
+@InterfaceType({
+  resolveType: value => {
+    if ("grades" in value) {
+      return "Student"; // schema name of the type as a string
+    }
+    return Person; // or the object type class
+  },
+})
+abstract class IPerson {
+  // ...
+}
+```
+
+However in case of interfaces, it might be a little bit more tricky than with unions, as we might not remember all the object types that implements this particular interface.
+
 ## Examples
 
-For more advanced usage examples of interfaces (and type inheritance), e.g. with query returning an interface type, got to [this examples folder](https://github.com/19majkel94/type-graphql/tree/master/examples/interfaces-inheritance).
+For more advanced usage examples of interfaces (and type inheritance), e.g. with query returning an interface type, go to [this examples folder](https://github.com/19majkel94/type-graphql/tree/master/examples/interfaces-inheritance).

docs/unions.md

@@ -4,7 +4,7 @@ title: Unions
 
 Sometimes our API has to be flexible and return a type that is not specific but one from a range of possible types. An example might be a movie site's search functionality: using the provided phrase we search the database for movies but also actors. So the query has to return a list of `Movie` or `Actor` types.
 
-Read more about the GraphQL union type in the [official docs](http://graphql.org/learn/schema/#union-types).
+Read more about the GraphQL Union Type in the [official GraphQL docs](http://graphql.org/learn/schema/#union-types).
 
 ## Usage
 
@@ -60,8 +60,29 @@ class SearchResolver {
 }
 ```
 
+## Resolving Type
+
 Be aware that when the query/mutation return type (or field type) is a union, we have to return a specific instance of the object type class. Otherwise, `graphql-js` will not be able to detect the underlying GraphQL type correctly when we use plain JS objects.
 
+However, we can also provide our own `resolveType` function implementation to the `createUnionType` options. This way we can return plain objects in resolvers and then determine the returned object type by checking the shape of the data object, e.g.:
+
+```typescript
+const SearchResultUnion = createUnionType({
+  name: "SearchResult",
+  types: [Movie, Actor],
+  // our implementation of detecting returned object type
+  resolveType: value => {
+    if ("rating" in value) {
+      return Movie; // we can return object type class (the one with `@ObjectType()`)
+    }
+    if ("age" in value) {
+      return "Actor"; // or the schema name of the type as a string
+    }
+    return undefined;
+  },
+});
+```
+
 **Et Voilà!** We can now build the schema and make the example query 😉
 
 ```graphql
@@ -80,3 +101,7 @@ query {
   }
 }
 ```
+
+## Examples
+
+More advanced usage examples of unions (and enums) are located in [this examples folder](https://github.com/19majkel94/type-graphql/tree/master/examples/enums-and-unions).