Plugin to generate a single schema file, to be consumed by Relay

[davidkus]

TLDR: Plugin to generate a single GraphQL schema file for the generated server, which can then be consumed by Relay.

In trying to get EntGo + gqlgen + Relay to work together, I ran into a few issues:

1. Problem: The EntGo gql plugin does not generate the type Mutation. It also only generates a strict set of Queries - and I had a need for some custom queries.
   Solution: Use multiple schema files, which gqlgen happily supports. In addition to the schema file generated by entgo, I included a hand-written one that contained the mutation definitions, as well as some custom queries by extending the Query type, e.g.:

extend type Query {
  viewer: User!
}

2. Problem: Relay only takes a single schema file as input.
   Solution: Concatenate my two schema files together, à la cat schema1.graphql schema2.graphql > combined_schema.graphql.
3. Problem: Relay has a concept of Client Schema Extensions, and they leverage GraphQL's extend type to achieve this. Long story short, Relay does not send anything defined in an extend type to the server. So, my custom queries were not being sent.
   Solution: Using gqlgen's plugin framework, generate a schema file based on all definitions known by the generated server. Leverages github.com/vektah/gqlparser - which is an existing dependency of gqlgen.

The Plugin:

package plugin

import (
	"os"

	"github.com/99designs/gqlgen/codegen"
	"github.com/99designs/gqlgen/plugin"
	"github.com/vektah/gqlparser/v2/formatter"
)

func New() plugin.Plugin {
	return &EffectiveSchemaPlugin{}
}

type EffectiveSchemaPlugin struct{}

func (m *EffectiveSchemaPlugin) Name() string {
	return "effectiveschemagen"
}

func (m *EffectiveSchemaPlugin) GenerateCode(data *codegen.Data) error {

	f, err := os.Create("./generated/schema.graphql")
	if err != nil {
		return err
	}
	defer f.Close()

	formatter := formatter.NewFormatter(f)
	formatter.FormatSchema(data.Schema)
	return nil
}

This will actually generate the "effective schema" - all extend types will be merged into the base type.

So these two files:

type Query {
  users: UserConnection!
}

extend type Query {
  viewer: User!
}

Are merged into this single file:

type Query {
  users: UserConnection!
  viewer: User!
}

Hopefully this can save someone from a headache when using gqlgen with Relay.