20 September 2018

Using Ecto's Virtual Fields with select_merge

Using Ecto’s virtual fields with select_merge

You may know that Ecto has a “virtual” field type that you can use to include pretty much anything in an Ecto struct. But did you know that you can also populate a virtual field directly in an Ecto Query?

Say I have a users table, and joined to that I have a posts table. The User has_many Posts. The User schema looks like this:

  schema "users" do
    field :name, :string
    has_many :posts, Post

And the Post schema looks like this:

  schema "posts" do
    field :date, :date
    belongs_to :user, User

And let’s say that in this app I’m writing, the date of a user’s first post is important and displayed various places. Today I learned that I can map the first post’s date directly onto the User struct. To do this, we have to add a virtual field to User, so that it looks like this:

  schema "users" do
    field :name, :string
    field :first_post_date, :date, virtual: true

    has_many :posts, Post


Then, I can use two awesome Ecto features to select the first post date and merge it into the User struct. Here’s an Ecto query to do just that!

        user in User,
        left_join: post in subquery(from p in Post, order_by: :date, limit: 1),
        on: post.user_id == user.id,
                select_merge: %{first_post_date: post.date}

The result is a list of User structs where the first_post date is populated (or is nil if the user hasn’t posted yet).

    first_post_date: ~D[2011-01-05],
    id: 2,
    name: "Tim",
    first_post_date: nil,
    id: 3,
    name: "Lurker McLurkerFace",

What strange magic happened here

The key components of Ecto that enable this are the following:

  • The virtual first_post_date field on User
  • The left_join: post in subquery(from p in Post, order_by: :date, limit: 1) that joins the user’s first post if it exists
  • The select_merge: %{first_post_date: post.date} that merges in the first_post_date value to the User struct.

What are other ways I could do this?

In Ecto:

  • I could preload the posts and have a function to select the first post’s date and merge it in separately.
  • I could select the first post date via a separate Ecto Query.

In the database:

  • I could put a first_post_date in the users table, and have a trigger that updates the field.
  • I could have a database view that contains first post using a similar join as above.

The cool thing about the solution above is that if I do choose to something in the database later, the only things that need to change are the field definition and the Ecto Query. The rest of the application can just keep using first_post_date like always!

Heads up! This article may make reference to the Gaslight team—that's still us! We go by Launch Scout now, this article was just written before we re-introduced ourselves. Find out more here.

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