How to deal with nulls in GraphQL schema

user_9162463 0
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I keep getting the "Cannot return null for non-nullable field Airline.id." error when FlightSchedule.operatingAirline is null (perfectly valid as per schema) and client queries for FlightSchedule.operatingAirline.id. How to fix this? Making Airline.id, Airline.code and Airline.name as nullable fixes this but is not the right way to solve this problem because if an Airline exist, these 3 fields will always exist too. Below is my schema:

type Airline {
    id: String!,
    code: String!,
    name: String!
}

type FlightSchedule {
    airline: Airline!
    operatingAirline: Airline
}

And below is my query:

  getFlightSchedules {
    airline
    {
      id
      code
      name
    }
    operatingAirline
    {
      id
      code
      name
    }
  }
apollo-server graphql
1 Answers

A field will resolve to null when an error is encountered while resolving it. This includes validation errors like the one you're encountering. From the spec:

If during ExecuteSelectionSet() a field with a non‐null fieldType throws a field error then that error must propagate to this entire selection set, either resolving to null if allowed or further propagated to a parent field.

If this occurs, any sibling fields which have not yet executed or have not yet yielded a value may be cancelled to avoid unnecessary work.

In other words, if a parent field is of a particular object type, and that type has a non-nullable field, and that field resolves to null, that parent field will also resolve to null. The parent field cannot return an object that is invalid (in this case because it had a non-null field return null), so the only thing it can do is return null. Of course, if the parent field itself is non-null, this behavior is propagated up the tree until a nullable field is finally encountered.

So, why are you getting that error? Because your resolver for operatingAirline is not returning null. It is returning some kind of object (either an incomplete airline object, an array, a string or something else) that GraphQL then effectively tries to coerce into the Airline type. The id field was requested, but it resolves to null based on the object returned by operatingAirline's resolver. Since the id was requested and returned null, the entire operatingAirline field fails validation and returns null.

user_60242200
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