Limit the number of preloaded items

Hello,

Having a relationship like this:

defmodule Cmr.Shelf.Book do
  # ...
  schema "shelf_books" do
    # ...
    has_many :keywords, Keyword
  end
end

defmodule Cmr.Shelf.Keyword do
  # ...
  schema "shelf_keywords" do
    # ...
    field :occurrences, :integer
    belongs_to :book, Book
  end
end

I want to build a query that brings a list of Shelf.Book’s. Since a book can have hundreds of keywords, I want to optimize the amount of data that I send to the client. The idea is to improve my query to only bring the n (let’s say 15) keywords with more occurrences.

So far I tried to do something in the lines of:

    # Define a query for keywords, limit the number to 15
    k_query = from k in Keyword,
      limit: 15,
      order_by: [desc: k.occurrences]

    # Query books, but using k_query to preload associations
    Repo.all from b in Book,
      limit: 10,
      preload: [keywords: ^k_query]

But this doesn’t work properly, and I suspect that it’s because the limit is being applied to the total number of keywords.

For example, this is the output of the query in JSON format, the first item in the list is coming with 15 keywords, while all the following ones have an empty array. I can confirm that all the books have associated keywords.

{
  "data": [{
    "title": "Pride and Prejudice",
    "keywords": [
      // 15 items here
    ],
    "id": 1,
    "gutenberg_id": 1342,
    "author": "Austen, Jane"
  }, {
    "title": "Alice's Adventures in Wonderland",
    "keywords": [
      // No items here
    ],
    "id": 2,
    "gutenberg_id": 11,
    "author": "Carroll, Lewis"
  }]
}

Any hint/pointer will be appreciated! Thanks!