I created this question in case anyone was curious on how to add union / Polymorphic types in Apollo. Hopefully this will make it easier for them.
In this example I wanted the response to either be a Worksheet
or ApiError
// typedefs.js
export default [`
schema {
query: Query
}
type Query {
worksheet(id: String!): Worksheet | Error
}
type Worksheet {
id: String!
name String
}
type ApiError {
code: String!
message: String!
}
`];
// resolvers.js
export default {
Query: {
worksheet(_, args, { loaders }) {
return loaders.worksheet.get(args.id).catch(() => {
// ApiError
return {
code: '1',
message: 'test'
}
});
}
}
};
// Express Server
import { graphqlExpress } from 'apollo-server-express';
import { makeExecutableSchema } from 'graphql-tools';
import typeDefs from './typedefs';
import resolvers from './resolvers';
...
app.post(
'/graphql',
graphqlExpress(req => ({
makeExecutableSchema({ typeDefs, resolvers }),
context: mkRequestContext(req.ctx, req.log),
formatError: formatGraphQLError(req.ctx, req.log)
}))
);
In GraphQL to add a union type in the typedefs you have to define the union
ie union WorksheetOrError = Worksheet | ApiError
union WorksheetOrError = Worksheet | ApiError
// typedefs.js
export default [
`
schema {
query: Query
}
type Query {
worksheet(id: String!): WorksheetOrError
}
union WorksheetOrError = Worksheet | ApiError
type Worksheet {
id: String!
name String
}
type ApiError {
code: String!
message: String!
}
`];
In the resolvers you have to define a resolver for the union type that has the property __resolveType . This will help tell the GraphQL executor which type the result is.
// resolvers.js
export default {
Query: {
worksheet() {
...
}
},
WorksheetOrError: {
__resolveType(obj) {
if (obj.id) {
return 'Worksheet';
}
if (obj.code) {
return 'ApiError';
}
return null;
}
},
};
To create a GraphQL Query in Apollo Client
// Your application code.
// This is my Worksheet Query in my React code.
const WorksheetQuery = gql`
query GetWorksheet($worksheetId: String!) {
worksheet(id: $worksheetId) {
... on Worksheet {
id
name
}
... on ApiError {
code
message
}
}
}
Now you can check the __typename
to check what type is in the response.
Note: For those who are wondering why I'm not using GraphQL errors . It's because Apollo doesn't seem to handle errors well when it encounters a graphQL error. So for a work around I'm trying to return a custom
ApiError
in my response.
There a few reasons why using a union with an error type is nice.
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