Step 5: Has One

View the Diff

In the last step, we introduced the concept of a “current position” to the model layer. Let’s now expose that relationship to the API.

The Rails Stuff 🚂

We already defined a Position.current scope that we’ll re-use - let’s just make a small tweak to support the opposite use case as well:

scope :current, ->(bool) {
  clause = { historical_index: 1 }
  bool ? where(clause) : where.not(clause)

The Graphiti Stuff 🎨

You might already have an idea how this might work from the prior step

  • we’ll use the params block to customize the relationship. The has_one macro ensures the result is treated as a single object and not an array.
# app/resources/employee_resource.rb
has_one :current_position, resource: PositionResource do
  params do |hash|
    hash[:filter][:current] = true

Which means we’ll have to implement that filter - re-using the ActiveRecord scope we already defined!

filter :current, :boolean do
  eq { |scope, value| scope.current(value) }

Digging Deeper 🧐

In this example, we’re able to return only a single record because we have a historical_index column. If this column didn’t exist - maybe we’re just ordering on created_at and taking the first record - we’d have a problem. What if we were loading 20 employees and wanted the current_position of each - what SQL would limit the resultset correctly?

We call this a faux has_one and there’s nothing easily done here. Graphiti will ensure only one record is returned by the API, but the query will take longer and loading extra records will eat memory. If there are lots of records in the association, look into adding a column like historical_index.