Prefer returning chainable ActiveRecord objects

One of the best parts about ActiveRecord is the chainable query interface:

Post.includes(:comments)
  .where(published: true)
  .where(author: Current.user)
  .order(:name)

To take advantage of this strength and give you flexibility in your code, always try to return chainable objects when querying data.

Usage

It’s common to extract complex queries as your application grows.

class SpecialOffer

  def self.find_eligible_products(store, shopper)
    return [] if store.restricted?

    store.products
      .where('price >= ?', 100)
      .select{ |p| shopper.can_order?(p) }
  end
end

@products = SpecialOffer.find_eligible_products(store, shopper)
#=> [#<Product:0x00007fb1719b7ec0>, #<Product:0x00007fb174744de8>, ...]

While this code may work, what happens if you need to order the @products in a certain way? Or add additional logic? Or lazy-load some associations?

In this case, the return type of our SpecialOffer method are arrays. We would have to switch to using Ruby array methods like sort and select and maybe accidentally introduce an N+1 bug if we need more data.

Let’s refactor this code to make it return chainable objects.

class SpecialOffer

  def self.find_eligible_products(store, shopper)
    return Product.none if store.restricted?

    product_ids = store.products
      .where('price >= ?', 100)
      .select{ |p| shopper.can_order?(p) }
      .map(&:id)

    Product.where(id: product_ids)
  end
end

@products = SpecialOffer.find_eligible_products(store, shopper)
#=> Product::ActiveRecord_Relation

First, we make use of the none query method: this returns an empty (but still chainable!) result. You can call ActiveRecord methods like order, includes, or where on this empty relation and it will simply return no results.

Second, instead of returning the result of our complex product query directly, we collect up the right products and then return “fresh” results for just those ids. While this does incur an additional database query, we can also manipulate the results as needed.

If we want to sort the results or load an association, we can do it in the database and not be worried about any existing conditions that were run as part of the computations.

@products = SpecialOffer.find_eligible_products(store, shopper)
  .includes(:variants)
  .order(:price)

@products = SpecialOffer.find_eligible_products(store, shopper)
  .joins(:sales)
  .where("sales.count > 15")
  .order(:sku)

I’ve found this pattern to be extremely helpful for pulling out complex queries, while still maintaining flexibility to massage the data into the correct shape.

Additional Resources

Rails API: ActiveRecord::QueryMethods#none

Rails Docs: Active Record Query Interface

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Comments

[Kieran Pilkington]

This is a good practice. I would recommend one addition though: only pull the columns from the DB that you need to fetch the right IDs, rather than everything. e.g.

Assuming shopper.can_order? is something like:

def can_order?(p)
  country == p.country
end

then we only need to fetch two things: The ID (which we want to return), and the country which is used in can_order?.

store.products
  .where('price >= ?', 100)
  .select(:id, :country)
  .select{ |p| shopper.can_order?(p) }
  .map(&:id)

[Francisco Quintero 🇨🇴]

Nice tip!