Pagi

this is an example of relay style cursor based pagination with elixir, phoenix, absinthe.

i'm sharing this to discuss about one possible problem in such condition:

• pagination of sorted data
• & the key used for sorting can be updated (e.g. total_post_views)
• if updates happens while you're navigating (it means query comes after an update)
• some of your data will not appear in your list (PROBLEM!)
• how to handle this?

To start:

• Install dependencies with mix deps.get
• Create and migrate your database with mix ecto.create && mix ecto.migrate
• Install Seed data of this project mix run priv/repo/seeds.exs
• Start Phoenix endpoint with mix phx.server

Graphiql:

Now you can visit localhost:4000/graph/graphiql from your browser.

Scenario of this project?

• there will be 10 posts and each has 100, 200, ..., 1000 total_post_views
• paginate first 5 trending posts (order_by desc: total_post_views)
• simulate! (most unpopular post will take the 2nd position)
• continue to paginate first 5 trending posts, after "YXJyYXljb25uZWN0aW9uOjQ=" (5th)
• you will see one post (which has been boosted) is not in your list (PROBLEM!)

What you can do?

Queries:

• trending_posts: list trending_posts (order_by desc: :total_post_views)

Mutations:

• reset: reset all data as same as seeds.exs
• simulate: simluate scenario: make 'most unpopular post' be a second one
• update_post: you can manually update post for testing

Examples:

Page 1:

{
  trendingPosts(first: 5) {
    edges {
      node {
        name
        totalPostViews
      }
      cursor
    }
  }
}

Page 2:

{
  trendingPosts(first: 5, after: "YXJyYXljb25uZWN0aW9uOjQ=") {
    edges {
      node {
        name
        totalPostViews
      }
      cursor
    }
  }
}

Simulate:

mutation {
  simulate
}

Reset:

mutation {
  reset
}

Update Post:

mutation {
  updatePost(id: 1, post: {totalPostViews: 100}) {
    name
    totalPostViews
  }
}