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Using Hasura in Apollo Federation

Motivation

Apollo federation lets users combine multiple GraphQL APIs (called subgraphs) into a unified API. We want Hasura users to be able to use Hasura to build one of the subgraphs and bring it into Apollo federation. This requires us to provide a way in the Hasura metadata to be able to produce a GraphQL schema that can work with Apollo federation.

Apollo Federation

There are two aspects to integrating with Apollo federation:

  • Being the left side of a relationship, where another Apollo subgraph will resolve the relationship
  • Being the right side of a relationship, which Apollo will use for resolving relationships from other Apollo subgraphs

For the first case, if we want a relationship from a Review object to a Product object coming from Apollo, we need to generate a schema like this:

type Review {
  id: Int!
  product: Product! # Note: in the database, this field would just be product_id
  description: String!
}

type Product @key(fields: "id", resolvable: false) {
  id: Int!
}

For the second case, if we want to allow a relationship from an Review object in Apollo to a Product object in Hasura, we need to generate a schema like this:

type Product @key(fields: "id") {
  id: Int!
  name: String!
}

query Query {
  _entities(representations: [_Any!]!): [_Entity]!
}

union _Entity = Product | ... # any other types that can be referenced from Apollo

Proposed Metadata Changes

The following changes in the metadata will allow us to achieve both these use cases.

Allow nesting NDC columns in OpenDD

First, we would need to allow mapping the columns of an NDC collection to a field nested within a sub-object within the OpenDD model's object type.

The current schema for fieldMapping in OpenDD will need to be changed to a oneOf to be able to support that.

So, for mapping an NDC collection for reviews with columns id, description, and product_id, to the desired GraphQL schema described in the Apollo Federation section, the mapping would look like this:

fieldMapping:
  id:
    column:
      name: id
  description:
    column:
      name: description
  product:
    nestedFieldMapping:
      id:
        column:
          name:
            product_id

Allow generating Apollo directives

The following metadata can be configured to generate the appropriate Apollo directives.

For generating the directives for left side of a relationship.

kind: ObjectType
version: v1
definition:
  name: Product
  fields:
    - name: id
      type: Int!
  graphql:
    typeName: Product
    apolloFederation:
      # generates the key directive for Product, with resolvable false if no model
      # is defined as the entitySource.
      keys:
        - fields:
          - id

For generating the types / directives for the right side of a relationship.

kind: Model
version: v1
definition:
  name: Products
  objectType: Product
  graphql:
    apolloFederation:
        # generates the key directive on `Product` and adds it to `_Entity` union
        entitySource: true