Overriding existing queries and mutations

While tailoring a new schema from existing GraphQL schemas using schema stitching, we can also override the existing queries and mutations. This is particularly useful when:

• Adding custom validation to the resolvers
• Data clean-up and case transformation

Adding custom checks

Let us consider a gaming room GraphQL API that serves the following schema.

# roottype Query {  game: game}

type game {  id: Int,  name: String,  type: String,  multiplayer: Boolean,  ip: String}

We will now add a custom validation to the game query such that this query passes through only after 17:00 (so that kids don’t game during the day ;-)).

1. Create a remote executable schema from the the existing game API.

2. Write a custom resolver to add custom validation to the game query.

3. Merge the executable game schema with our custom resolvers:

4. Serve the new schema:

Data cleanup

We might want to sanitize or modify some data in our GraphQL server before we delegate it to some underlying GraphQL API. This can be achieved with the help of a custom resolver as illustrated below.

Let us consider a simple userprofile schema where the insert_user mutation has to be delegated to an underlying schema. Before delegating, the email in the data has to be converted to lowercase.

The algorithm is as follows:

1. Create a remote executable schema with the makeRemoteExecutableSchema() function:

2. Create a custom resolver for converting the email to lowercase.

3. Merge the executable schema with the resolver using the mergeSchemas function:

4. Serve the merged schema with a new instance of the apollo-server: