Unique constraints in GraphQL

This tutorial assumes that you have successfully completed the Dashboard quick start tutorial, and that you still have the Fauna Dashboard open in a browser tab/window, on the GraphQL Playground screen.

If your Dashboard session has expired:

  1. Log in again.

  2. Select the graphql database.

  3. Click the GRAPHQL button in the left navigation.

This tutorial explores the @unique directive, which allows you to add constraints to your GraphQL schema. When you add @unique to a field definition, Fauna handles the creation of an index that enforces this constraint.


Let’s define a simple user type, with a uniqueness constraint on the username, and see what happens when the constraint is triggered.

  1. Create a new schema file

    Create the file schema-unique.gql with the following content (or download it here):

    type User {
      username: String! @unique
  2. Import the new GraphQL schema into Fauna

    Click the MERGE SCHEMA button in the GraphQL Playground screen (in your browser), which opens your browser’s file selector. Select the schema-unique.gql file, and click the file selector’s Open button.

    This new schema only updates collections (and associated indexes) with the same name. Any other collections are unaffected.
  3. Inspect the constraint

    Let’s inspect the constraint via Fauna Shell.

    Open a terminal and run:

    fauna shell graphql

    After Shell starts, run the following query:


    You should see output similar to:

    { ref: Index("unique_User_username"),
      ts: 1559780318060000,
      active: true,
      partitions: 1,
      name: 'unique_User_username',
      source: Collection("User"),
      data: { gql: { ts: Time("2019-06-06T00:18:37.979330Z") } },
      values: [],
      terms: [ { field: [ 'data', 'username' ] } ],
      unique: true }

    The unique field in the result is set to true, and the terms specify which field(s) need to be unique, i.e. data/username.

  4. Add a user

    Copy the following GraphQL query:

    mutation CreateAUser {
       createUser(data: {
         username: "Alice"
       }) {

    Then click the "new tab" + button on the GraphQL Playground screen in your browser (at the top left, just right of the last query tab). Paste the query into the left panel, and click the "Play" button. The query should execute and the response should appear in the right panel:

      "data": {
        "createUser": {
          "username": "Alice"
  5. Add a duplicate user

    Let’s see what happens when a duplicate user is added. Simply click the "Play" button a second time. The query should execute, and an error response should appear in the right panel:

      "errors": [
          "message": "Instance is not unique.",
          "extensions": {
            "code": "instance not unique"


This tutorial has demonstrated that the @unique directive can be used to apply a uniqueness constraint to a field in your GraphQL schema.

For more information, see the @unique reference.

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