GraphQL Field Collection

Clone the code (optional)

The code for this tutorial is available under , and tagged (using Git) in each step. If you want to skip the basic setup and start with the initial version of the GraphQL server, you can clone the repository and checkout as follows:

The “N+1 problem” in GraphQL means that a server executes unnecessary database queries to get node associations (i.e. edges) when it can be avoided. The number of queries that potentially executed (N+1) is a factor of the number of the nodes returned by the root query, their associations, and so on recursively. That means, this can be a very big number (much bigger than N+1).

Let’s try to explain this with the following query:

  1. query {
  2.     users(first: 50) {
  3.         edges {
  4.             node {
  5.                 photos {
  6.                     link
  7.                 }
  8.                 posts {
  9.                     content
  10.                     comments {
  11.                         content
  12.                     }
  13.             }
  14.         }
  15.     }
  16. }

In the query above, we want to fetch the first 50 users with their photos and their posts including their comments.

In the naive solution (the problematic case), a server will fetch the first 50 users in 1 query, then, for each user will execute a query for getting their photos (50 queries), and another query for getting their posts (50). Let’s say, each user has exactly 10 posts. Therefore, For each post (of each user), the server will execute another query for getting its comments (500). That means, we have 1+50+50+500=601 queries in total.

The Ent extension for field collection adds support for automatic GraphQL fields collection for associations (i.e. edges) using . That means, if a query asks for nodes and their edges, entgql will automatically add With steps to the root query, and as a result, the client will execute constant number of queries to the database - and it works recursively.

That means, in the GraphQL query above, the client will execute 1 query for getting the users, 1 for getting the photos, and another 2 for getting the posts, and their comments (4 in total!). This logic works both for root queries/resolvers and for the node(s) API.

Before we go over the example, we change the to run in debug mode in the Todos resolver and restart our GraphQL server:

  1. func (r *queryResolver) Todos(ctx context.Context, after *ent.Cursor, first *int, before *ent.Cursor, last *int, orderBy *ent.TodoOrder) (*ent.TodoConnection, error) {
  2. -   return r.client.Todo.Query().
  3. +   return r.client.Debug().Todo.Query().
  4.         Paginate(ctx, after, first, before, last,
  5.             ent.WithTodoOrder(orderBy),
  6.         )
  7. }

Then, we execute the GraphQL query from the , but we add the parent edge to the result:

We check the process output, and we’ll see that the server executed 11 queries to the database. 1 for getting the last 10 todo items, and another 10 for getting the parent of each item:

  1. SELECT DISTINCT `todos`.`id`, `todos`.`text`, `todos`.`created_at`, `todos`.`status`, `todos`.`priority` FROM `todos` ORDER BY `id` ASC LIMIT 11
  2. SELECT DISTINCT `todos`.`id`, `todos`.`text`, `todos`.`created_at`, `todos`.`status`, `todos`.`priority` FROM `todos` JOIN (SELECT `todo_parent` FROM `todos` WHERE `id` = ?) AS `t1` ON `todos`.`id` = `t1`.`todo_parent` LIMIT 2
  3. SELECT DISTINCT `todos`.`id`, `todos`.`text`, `todos`.`created_at`, `todos`.`status`, `todos`.`priority` FROM `todos` JOIN (SELECT `todo_parent` FROM `todos` WHERE `id` = ?) AS `t1` ON `todos`.`id` = `t1`.`todo_parent` LIMIT 2
  4. SELECT DISTINCT `todos`.`id`, `todos`.`text`, `todos`.`created_at`, `todos`.`status`, `todos`.`priority` FROM `todos` JOIN (SELECT `todo_parent` FROM `todos` WHERE `id` = ?) AS `t1` ON `todos`.`id` = `t1`.`todo_parent` LIMIT 2
  5. SELECT DISTINCT `todos`.`id`, `todos`.`text`, `todos`.`created_at`, `todos`.`status`, `todos`.`priority` FROM `todos` JOIN (SELECT `todo_parent` FROM `todos` WHERE `id` = ?) AS `t1` ON `todos`.`id` = `t1`.`todo_parent` LIMIT 2
  6. SELECT DISTINCT `todos`.`id`, `todos`.`text`, `todos`.`created_at`, `todos`.`status`, `todos`.`priority` FROM `todos` JOIN (SELECT `todo_parent` FROM `todos` WHERE `id` = ?) AS `t1` ON `todos`.`id` = `t1`.`todo_parent` LIMIT 2
  7. SELECT DISTINCT `todos`.`id`, `todos`.`text`, `todos`.`created_at`, `todos`.`status`, `todos`.`priority` FROM `todos` JOIN (SELECT `todo_parent` FROM `todos` WHERE `id` = ?) AS `t1` ON `todos`.`id` = `t1`.`todo_parent` LIMIT 2
  8. SELECT DISTINCT `todos`.`id`, `todos`.`text`, `todos`.`created_at`, `todos`.`status`, `todos`.`priority` FROM `todos` JOIN (SELECT `todo_parent` FROM `todos` WHERE `id` = ?) AS `t1` ON `todos`.`id` = `t1`.`todo_parent` LIMIT 2
  9. SELECT DISTINCT `todos`.`id`, `todos`.`text`, `todos`.`created_at`, `todos`.`status`, `todos`.`priority` FROM `todos` JOIN (SELECT `todo_parent` FROM `todos` WHERE `id` = ?) AS `t1` ON `todos`.`id` = `t1`.`todo_parent` LIMIT 2
  1. func (Todo) Edges() []ent.Edge {
  2.     return []ent.Edge{
  3.         edge.To("parent", Todo.Type).
  4. +           Annotations(entgql.Bind()).
  5.             Unique().
  6.             From("children").
  7. +           Annotations(entgql.Bind()),
  8.     }
  9. }
info" class="reference-link">Field Collection - 图3info

The Bind() annotation is now enabled by default, use Unbind() to opt-out.

After adding these annotations, entgql will do the binding mentioned in the above. Additionally, it will also generate edge-resolvers for the nodes under the edge.go file:

Let’s run the code generation again and re-run our GraphQL server:

  1. go generate ./...
  2. go run ./cmd/todo

If we check the process’s output again, we will see that this time the server executed only two queries to the database. One, in order to get the last 10 todo items, and a second one for getting the parent-item of each todo-item that was returned in the first query.

  1. SELECT DISTINCT `todos`.`id`, `todos`.`text`, `todos`.`created_at`, `todos`.`status`, `todos`.`priority`, `todos`.`todo_parent` FROM `todos` ORDER BY `id` DESC LIMIT 11
  2. SELECT DISTINCT `todos`.`id`, `todos`.`text`, `todos`.`created_at`, `todos`.`status`, `todos`.`priority` FROM `todos` WHERE `todos`.`id` IN (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)

If you’re having troubles running this example, go to the first section, clone the code and run the example.