Cannot return null for non-nullable field Subscription.greeting

[jmc420]

I am getting "Cannot return null for non-nullable field Subscription.greeting" returned to Postman when I invoke a subscription:

Schema looks like this:

const gql = `
    type Query {
        sayHello: String!
      }
      
      type Mutation {
        sendMessage(message:String!):String !
      }
      
      type Subscription {
          greeting: String!
      }
    `;

The resolver looks like this:

const rootValue = {
    subscription: {
        greeting: async function* () {
            for (const hi of ['Hi', 'Bonjour', 'Hola', 'Ciao', 'Zdravo']) {
                yield { greetings: hi };
            }
        }
    }
}

The full code of the example can be found here:

https://github.com/jmc420/graphql_examples/blob/main/graphql-ws/src/server.ts

[enisdenjo]

Try changing your root resolver to use the subscribe field instead:

const rootValue = {
    subscription: {
-        greeting: async function* () {
-            for (const hi of ['Hi', 'Bonjour', 'Hola', 'Ciao', 'Zdravo']) {
-                yield { greetings: hi };
-            }
-        }
+        greeting: {
+            subscribe: async function* () {
+                for (const hi of ['Hi', 'Bonjour', 'Hola', 'Ciao', 'Zdravo']) {
+                    yield { greetings: hi };
+                }
+            }
+        }
    }
}

[jmc420]

yes removing the subscribe fixes it. The resolvers for graphql-http don't work in the same way as the resolvers for graphql-ws.

The resolvers for graphql-ws are:

const rootValue = {
    query: {
        sayHello: () => {
            return "Hello"
        }
    },
    mutation: {
        sendMessage: ({ message }) => {
            return "You said " + message
        }
    },
    subscription: {
        greeting: async function* () {
            for (const hi of ['Hi', 'Bonjour', 'Hola', 'Ciao', 'Zdravo']) {
                yield { greeting: hi };
            }
        }
    }
}

The resolvers for graphql-http are:

const resolvers = {
    sayHello: () => {
        return "Hello"
    },
    sendMessage: ({ message }) => {
        return "You said " + message
    }
}

it's rather confusing!

[enisdenjo]

You should always use the GraphQL schema and have all resolvers contained within - don't use the rootValue. I heavily recommend graphql-tools for assembling the schema. I created a PR for you: jmc420/graphql_examples#1.

[jmc420]

Thanks for the pull request. Graphql tools makes things a lot clearer.

Curiously though the query and the subscription now work, there is a problem with the mutation

mutation SendMessage {
    sendMessage(message: "Hello again")
}

now generates the error:

"message": "Cannot destructure property 'message' of 'undefined' as it is undefined.",

I added a debug-server command to scripts in package.json. When I put a break point in the body of sendMessage, the breakpoint was not triggered when there was a destructuring parameter; it never reached the code because of the destructuring problem. If I removed the destructuring parameter, the breakpoint was triggered. If I add a parameter to the query, I get the same problem.

Do you think the destructuring problem is within graphql-tools or in graphql-http?

[enisdenjo]

Do you think the destructuring problem is within graphql-tools or in graphql-http?

Neither, the problem is in your code here. The field args are the second argument of the resolver. Please read the graphql-tools documentation about schema generation.

Mutation: {
-    sendMessage: ({ message }) => {
+    sendMessage: (_, { message }) => {
        return "You said " + message
    }
},

[jmc420]

thanks that fixes it.

[enisdenjo]

From #460 (reply in thread)

Its was because of yielding the field "greetings" while the actual field is called just "greeting" (singular).

const rootValue = {
    subscription: {
        greeting: async function* () {
            for (const hi of ['Hi', 'Bonjour', 'Hola', 'Ciao', 'Zdravo']) {
-                yield { greetings: hi };
+                yield { greeting: hi };
            }
        }
    }
}