GraphQL update mutation returns null when patch includes unchanged @id value

[tonisole]

What happened?

When performing an update mutation in Dgraph GraphQL, if the mutation's set patch includes a field annotated with @id (for example, name: String! @id) and the value is unchanged (the same as the existing node), the mutation response is { "data": { "update<Type>": null } } with no errors array. Other fields in the patch are skipped and not updated. If the @id value is actually changed, or the @id field is omitted entirely, the mutation works as expected.

This bug can be reliably triggered with the following minimal schema and mutation sequence:

type Thing {
  id: ID!
  code: String! @id
  note: String
}

1. Add a node:

mutation {
  addThing(input: [{ code: "CODE1", note: "Initial note" }]) {
    thing { id code note }
  }
}

2. Update with unchanged @id (BUG):

mutation {
  updateThing(input: { filter: { code: ["CODE1"] }, set: { code: "CODE1", note: "Updated note" } }) {
    thing { id code note }
  }
}

Expected: The note field should be updated.
Actual: The mutation response is:

{ "data": { "updateThing": null } }

3. Update with changed @id (works):

mutation {
  updateThing(input: { filter: { code: ["CODE1"] }, set: { code: "CODE2", note: "Changed code" } }) {
    thing { id code note }
  }
}

The node is updated and the code changes to CODE2.

What did you expect to happen?

If the @id value is unchanged, the mutation should return a payload with numUids and update other fields normally. If no actual change is made, it should return numUids: 0 with the payload, not null. The current null response makes it hard to distinguish from authorization failure.

Reproduction steps

1. Deploy the schema above.
2. Add a node with a value for the @id field.
3. Attempt to update only a non-@id field, including the unchanged @id in the patch.
4. Observe the null response.

Impact

• Mutations that include an unchanged @id field fail silently (null response, no errors).
• Blocks batching updates when unchanged @id is present.
• Forces client logic to strip unchanged @id fields before sending update mutations.

Workaround

Only include the @id field in set when its value is genuinely changing. Filter out unchanged @id fields before sending update mutations.

Environment

• Dgraph version: v25.0.0
• GraphQL endpoint: /graphql

Additional context

This is not an authorization issue. Mutations succeed if the @id field is omitted or genuinely changed, and other mutations with the same token work. Presence of unchanged @id alone triggers the bug.

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Please investigate and advise on a fix.