Don't use this:

    scope :red, where(color: 'red')
    default_scope where(color: 'red')

Use this:

    scope :red, -> { where(color: 'red') }
    default_scope { where(color: 'red') }

The former has numerous issues. It is a common newbie gotcha to do
the following:

    scope :recent, where(published_at: Time.now - 2.weeks)

Or a more subtle variant:

    scope :recent, -> { where(published_at: Time.now - 2.weeks) }
    scope :recent_red, recent.where(color: 'red')

Eager scopes are also very complex to implement within Active
Record, and there are still bugs. For example, the following does
not do what you expect:

    scope :remove_conditions, except(:where)
    where(...).remove_conditions # => still has conditions