How to use Mongoose with GraphQL and DataLoader?

Keep your mongoose schema in a separate module. You don't want to create your schema each request -- just the first time the module is imported.

const userSchema = new Schema({  name: String,  friendIds: [String]})const User = mongoose.model("User", userSchema)module.exports = { User }

If you want, you can also export a function that creates your loader in the same module. Note, however, that we do not want to export an instance of a loader, just a function that will return one.

// ...const getUserLoader = () => new DataLoader((userIds) => {  return User.find({ _id: { $in: userIds } }).execute()})module.exports = { User, getUserLoader }

Next, we want to include our loader in the context. How exactly this is done will depend on what library you're using to actually expose your graphql endpoint. In apollo-server, for example, context is passed in as part of your configuration.

new ApolloServer({    typeDefs,    resolvers,    context: ({ req }) => ({        userLoader: getUserLoader()    }),})

This will ensure that we have a fresh instance of the loader created for each request. Now, your resolvers can just call the loader like this:

const resolvers = {  Query: {    users: async (root, args, { userLoader }) => {      // Our loader can't get all users, so let's use the model directly here      const allUsers = await User.find({})      // then tell the loader about the users we found      for (const user of allUsers) {        userLoader.prime(user.id, user);      }      // and finally return the result      return allUsers    }  },  User: {    friends: async (user, args, { userLoader }) => {      return userLoader.loadMany(user.friendIds)    },  },}