GraphQL Error Propagation

Before returning query and mutation results, Dgraph uses the types in the schema to apply GraphQL value completion and error handling. That is, null values for non-nullable fields, e.g. String!, cause error propagation to parent fields.

In short, the GraphQL value completion and error propagation mean the following.

• Fields marked as nullable (i.e. without !) can return null in the json response.
• For fields marked as non-nullable (i.e. with !) Dgraph never returns null for that field.
• If an instance of type has a non-nullable field that has evaluated to null, the whole instance results in null.
• Reducing an object to null might cause further error propagation. For example, querying for a post that has an author with a null name results in null: the null name (name: String!) causes the author to result in null, and a null author causes the post (author: Author!) to result in null.
• Error propagation for lists with nullable elements, e.g. friends [Author], can result in nulls inside the result list.
• Error propagation for lists with non-nullable elements results in null for friends [Author!] and would cause further error propagation for friends [Author!]!.

Note that, a query that results in no values for a list will always return the empty list [], not null, regardless of the nullability. For example, given a schema for an author with posts: [Post!]!, if an author has not posted anything and we queried for that author, the result for the posts field would be posts: [].

A list can, however, result in null due to GraphQL error propagation. For example, if the definition is posts: [Post!], and we queried for an author who has a list of posts. If one of those posts happened to have a null title (title is non-nullable title: String!), then that post would evaluate to null, the posts list can’t contain nulls and so the list reduces to null.