Example 3: Merging schemas with naming conflicts for types

Consider the following two schemas:

Schema A:

const typeDefs = `
  type Query {
    greeting: Greeting
  }

  type Greeting {
    text: String
  }`

const resolvers = {
  Query: {
    greeting: () => ({text: null})
  },
  Greeting: {
    text: () => 'Hello'
  }
}

const schemaA = makeExecutableSchema({ typeDefs, resolvers })

Schema B:

const typeDefs = `
  type Query {
    greeting: Greeting
  }

  type Greeting {
    text: String
  }`

const resolvers = {
  Query: {
    greeting: () => ({text: null})
  },
  Greeting: {
    text: root => 'A fine day, good sir!'
  }
}

const schemaB = makeExecutableSchema({ typeDefs, resolvers })

Both schemas are identical and thus have a conflict for the Greeting type. To resolve this situation, there are three options.

Option 1: Not explicitly resolve the conflict (use mergeSchema's default)

When not providing any additional information to mergeSchemas, it will by default pick the implementation of the last schema that was passed to it:

const { mergeSchemas } = require('graphql-tools')
const { schemaA, schemaB } = require('./schemas')

const mergedSchema = mergeSchemas({
  schemas: [schemaA, schemaB]
})

In this case mergedSchema will adopt the behaviour from schemaB as it was passed last into mergeSchemas.

Option 2: Write custom resolver

Whenevery mergeSchemas is used, you can provide a resolvers object containing explicit information about how the available fields are to be resolved (just like with makeExecutableSchema):

const { mergeSchemas } = require('graphql-tools')
const { schemaA, schemaB } = require('./schemas')

const mergedSchema = mergeSchemas({
  schemas: [schemaA, schemaB],
  resolvers: mergeInfo => ({
    Query: {
      greeting: () => ({text: null})
    },
    Greeting: {
      text: root => 'Hey'
    }
  })
})

Option 3: Use onTypeConflict

The last option is to use the onTypeConflict argument to determine which type should be used:

const { mergeSchemas } = require('graphql-tools')
const { schemaA, schemaB } = require('./schemas')

const mergedSchema = mergeSchemas({
  schemas: [schemaA, schemaB],
  onTypeConflict: (left, right) => right // default: left
})