Over the last few days, I’ve broken up pagination into a few mini-topics, each of which plays an important part in designing a good pagination system in GraphQL:

• cursor and limit arguments into our query resolvers

• returning Connections rather than lists directly. Allows us to:

  • Return both edges and nodes (can put metadata on edges).

  • Return a pageInfo object that tells the client:

    • cursor

    • hasNextPage

• using opaque cursors

  • to make it clear to the client not to depend on any particular type of data format on the cursor

  • The cursor is just a cursor to the client

  • Allows us to re-use the PageInfo type, as it just takes a cursor string and doesn’t care what the underlying format of the cursor is (when the resolver has decoded it into a createdAt date, for example).

So, on this lovely Friday morning in Burma, I’d like to just go through the exercise of implementing pagination all the way through, this time on Comments that belong to a Post.

Each Post can have multiple comments that belong to it. We want to allow the client to say:

• “Give me the first 3 comments that belong to this post”

• “Ok, the user wants to see more comments. Give me the next 3 comments. Here’s the last comment they saw, so start after that.”

Let’s get started.

Schema

• Add the PostCommentConnection to our schema.

  • Holds the data + pageInfo returned by the query

• Add the PostCommentEdge to our schema. This holds:

  • Edges

    • Nodes

    • Optional metadata on the edge (data relevant to the relationship itself, rather than the node)

• The comments field on our root Posts type should now take two arguments (cursor and limit) and should return a PostCommentConnection object rather than a list of Comment objects.

Resolver

Even though we’re building pagination for comments, we’re not working on the comment resolvers themselves, we’re actually working on the comment field resolver on the root Post object.

When the client is asking the API to describe a given post object, we want the client to optionally provide pagination arguments like this:

So in our Post type resolvers, we work inside the custom resolver for the Comment field:

And that’s all the server code we need to (get to?) write.

We can then test it out in GraphQL Playground and see that our new comment pagination system works swimmingly.

Happy Friday!

Sources:

The Road To GraphQL

GraphQL Docs: Pagination

Apollo Docs: Pagination

Apollo Blog: Explaining GraphQL Connections