Why does a GraphQL query return null?

Question

I have an graphql/apollo-server/graphql-yoga endpoint. This endpoint exposes data returned from a database (or a REST endpoint or some other service).

I know my data source is returning the correct data -- if I log the result of the call to the data source inside my resolver, I can see the data being returned. However, my GraphQL field(s) always resolve to null.

If I make the field non-null, I see the following error inside the errors array in the response:
Cannot return null for non-nullable field

Why is GraphQL not returning the data?

Ritesh swamy · Accepted Answer

There's two common reasons your field or fields are resolving to null: 1) returning data in the wrong shape inside your resolver; and 2) not using Promises correctly. Note: if you're seeing the following error: Cannot return null for non-nullable field the underlying issue is that your field is returning null. You can still follow the steps outlined below to try to resolve this error. The following examples will refer to this simple schema: type Query { post(id: ID): Post posts: [Post] } type Post { id: ID title: String body: String } Returning data in the wrong shape Our schema, along with the requested query, defines the "shape" of the data object in the response returned by our endpoint. By shape, we mean what properties objects have, and whether those properties' values' are scalar values, other objects, or arrays of objects or scalars. In the same way a schema defines the shape of the total response, the type of an individual field defines the shape of that field's value. The shape of the data we return in our resolver must likewise match this expected shape. When it doesn't, we frequently end up with unexpected nulls in our response.

GraphQL

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