useQuery returns undefined, But returns data on gql playground

问题

"@apollo/react-hooks": "^3.1.3",
"apollo-client": "^2.6.8",

Apollo client return undefined on react app but return the data on gql playground, I don't understand why don't it works on client-side but works on graphql playground.

Schema

I have defined union for user query for error handling.

type Query {
  user(id: ID!): UserReturn!
}

union UserReturn = User | Error

type User {
  id: ID!
  username: String!
  email: String
  profileUrl: String
  createdAt: Date
  ads: [Doc!]!
}


type Error {
  message: String
  code: ID
}

Query resolver

 async user(_, { id }, { User }) {
    console.log('query - User')
    try {
      await delay(1000 * 3)
      const user = await User.findById(id).populate('userData')
      console.log(user)
      if (!user) return {
        __typename: 'Error',
        message: 'User not found.',
        code: id
      }

      const { _id: id, username, email, createdAt, userData: { profileUrl } } = user

      console.log(username)
      return {
        __typename: 'User',
        id,
        username,
        email,
        createdAt,
        profileUrl
      }
    } catch (err) {
      console.log(err)
      return {
        __typename: 'Error',
        message: 'Something went wrong while getting user.',
        code: LogBack(err, `query/user?id=${id}`, __filename)
      }
    }
  }

When querying on gql playground

on graphql playground, query works.

On the client-side

 const { data } = useQuery(
    gql`query user($id: ID!) {
      user(id: $id) {
        __typename
        ... on User {
          id
          username
          email
          profileUrl
          createdAt
          # ads
        }
        ... on Error {
          message
          code
        }
      }
    }
    `,
    {
      variables: {
        id: userId
      }
    }
  );

  console.log(data) // undefined

useQuery runs but returns undefiend.

回答1:

something that might help, you know where you call {data} you can also look for error and console.log('Error:',error)

check the apollo client query docs

something like this , and look at the error message, it should help !

import { useQuery } from '@apollo/react-hooks';
import gql from 'graphql-tag';

const GET_GREETING = gql`
  query getGreeting($language: String!) {
    greeting(language: $language) {
      message
    }
  }
`;

function Hello() {
    const { loading, error, data } = useQuery(GET_GREETING, {
    variables: { language: 'english' },
  });
  if (loading) return 'Loading...';
  if (error) return `Error! ${error.message}`;
  return <h1>Hello {data.greeting.message}!</h1>;
}

回答2:

I'm able to query the ads by adding IntrospectionFragmentMatcher to my apollo client setup

This helps me to fix

Here how I fix the issue.

import { InMemoryCache, IntrospectionFragmentMatcher } from "apollo-cache-inmemory";

const fragmentMatcher = new IntrospectionFragmentMatcher({
  introspectionQueryResultData: {
    __schema: {
      types: [
        {
          "kind": "UNION",
          "name": "AdsReturn",
          "possibleTypes": [
            {
              "name": "Ads"
            },
            {
              "name": "Error"
            }
          ]
        },
      ],
    },
  }
});


export default new ApolloClient({
  link: ApolloLink.from([errorLink, authLink, httpLink]),
  cache: new InMemoryCache({ fragmentMatcher }),
});

来源：https://stackoverflow.com/questions/59920824/usequery-returns-undefined-but-returns-data-on-gql-playground