How to use dataloader?

Question

Im trying to figure this out.

I want to get all my users from my database, cache them and then when making a new request I want to get those that Ive cached + new ones that have been created.

So far:

const batchUsers = async ({ user }) => {
  const users = await user.findAll({});

  return users;
};

const apolloServer = new ApolloServer({
  schema,
  playground: true,
  context: {
    userLoader: new DataLoader(() => batchUsers(db)),// not sending keys since Im after all users
  },
});

my resolver:

    users: async (obj, args, context, info) => {
      return context.userLoader.load();
}

load method requiers a parameter but in this case I dont want to have a specific user I want all of them.

I dont understand how to implement this can someone please explain.

Daniel Rearden · Accepted Answer · 2020-04-16T23:11:19.327

1

If you're trying to just load all records, then there's not much of a point in utilizing DataLoader to begin in. The purpose behind DataLoader is to batch multiple calls like load(7) and load(22) into a single call that's then executed against your data source. If you need to get all users, then you should just call user.findAll directly.

Also, if you do end up using DataLoader, make sure you pass in a function, not an object as your context. The function will be ran on each request, which will ensure you're using a fresh instance of DataLoader instead of one with a stale cache.

context: () => ({
  userLoader: new DataLoader(async (ids) => {
    const users = await User.findAll({
      where: { id: ids }
    })

    // Note that we need to map over the original ids instead of
    // just returning the results of User.findAll because the
    // length of the returned array needs to match the length of the ids
    return ids.map(id => users.find(user => user.id === id) || null)
  }),
}),

Note that you could also return an instance of an error instead of null inside the array if you want load to reject.

edited Apr 16 '20 at 23:11

answered Apr 16 '20 at 19:00

Daniel Rearden

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Thanks for the explenation. Jump started my brain. I do realize that I chose a poor example with fetching all users. For clarity sake, would you mind adding an example where batching like you explain above would be useful. Again thanks alot! – CodingLittle Apr 16 '20 at 22:56
Edited answer to include full example – Daniel Rearden Apr 16 '20 at 23:11
Thank you! I was probably a bit unclear in the extra edit. Was hoping to see a scenario where you actually use the dataloader. My example with fecthing users was rather poor and was thinking perhaps you could share a scenario where you use it. Accepting the answer, if the extra request is not to much i would be greatful! – CodingLittle Apr 17 '20 at 08:28

score 1 · Answer 2 · answered Apr 16 '20 at 22:54

Took me a while but I got this working:

const batchUsers = async (keys, { user }) => {
  const users = await user.findAll({
    raw: true,
    where: {
      Id: {
        // @ts-ignore
        // eslint-disable-next-line no-undef
        [op.in]: keys,
      },
    },
  });

  const gs = _.groupBy(users, 'Id');
  return keys.map(k => gs[k] || []);
};

const apolloServer = new ApolloServer({
  schema,
  playground: true,
  context: () => ({
    userLoader: new DataLoader(keys => batchUsers(keys, db)),
  }),
});

resolver:

  user: {
    myUsers: ({ Id }, args, { userLoader }) => {
      return userLoader.load(Id);
    },
  },

playground:

{users
{Id
myUsers
{Id}}
}

playground explained:

users basically fetches all users and then myusers does the same thing by inhereting the id from the first call.

I think I choose a horrible example here since I did not see any gains in performence by this. I did see however that the query turned into:

SELECT ... FROM User WhERE ID IN(...)

How to use dataloader?

2 Answers2