Field Selections on Objects, Interfaces, and Unions Types

Formal Specification

• For each selection in the document.
• Let fieldName be the target field of selection
• fieldName must be defined on type in scope

Explanatory Text

The target field of a field selection must be defined on the scoped type of the selection set. There are no limitations on alias names.

For example the following fragment would not pass validation:

fragment fieldNotDefined on Dog {
  meowVolume
}

fragment aliasedLyingFieldTargetNotDefined on Dog {
  barkVolume: kawVolume
}

For interfaces, direct field selection can only be done on fields. Fields of concrete implementors are not relevant to the validity of the given interface‐typed selection set.

For example, the following is valid:

fragment interfaceFieldSelection on Pet {
  name
}

and the following is invalid:

fragment definedOnImplementorsButNotInterface on Pet {
  nickname
}

Because unions do not define fields, fields may not be directly selected from a union‐typed selection set, with the exception of the meta‐field__typename. Fields from a union‐typed selection set must only be queried indirectly via a fragment.

For example the following is valid:

fragment inDirectFieldSelectionOnUnion on CatOrDog {
  __typename
  ... on Pet {
    name
  }
  ... on Dog {
    barkVolume
  }
}

But the following is invalid:

fragment directFieldSelectionOnUnion on CatOrDog {
  name
  barkVolume
}