How to add Ecto.Query.API count() to Repo.preload?

Phoenix Forum Questions / Help
1

Hi,

I’m trying to preload some data to my user and I would like to add number of user notifications to the preload, not the notifications itself, just the count.

query # fetches the user
|> Repo.one()
|> Repo.preload([:profile, notifications: from(n in Notification, group_by: n.id, select: count(n.id))])

With this query I get one entry inside the list for each notification.

#MyApp.Accounts.User<
  __meta__: #Ecto.Schema.Metadata<:loaded, "users">,
  id: 6,
  ...
  notifications: [1, 1, 1, 1]

How do I count the notifications with the db so I would get:

#MyApp.Accounts.User<
  __meta__: #Ecto.Schema.Metadata<:loaded, "users">,
  id: 6,
  ...
  notifications: [4]

Thanks

2

I’d suggest using a virtual fields and select the count instead of kinda abusing preloads (which are a assoc/embed thing) for that.

3

Do you have some example maybe? I’ve used virtual fields with forms in the past but I don’t really understand how am I supposed to use them here.

And just to clarify, I used preload here to avoid making another query to the db just for counting notifications.

4

You are making another query here. Preloads use separate queries unless you bind them using joined data in queries.

defmodule Example do
  use Ecto.Schema
  import Ecto.Query
  
  schema "examples" do
    field :computed, :integer, virtual: true
  end
  
  def with_computed do
    from e in Example, select_merge: %{computed: 10}
  end
end
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