This post is obvious to any advanced Rails user (hell even any middling Rails user) but I know there are some readers who aren't that sophisticated so here goes.

A scope is a Rails database query that operates much like a class method only it is much, much simpler to define. Here's a class method that finds the active elements in a table called habits:

def active
  Habit.where(active: true)
end

Now that's not bad but a Rails scope looks like this:

scope :active, -> { where(active: true) }

But the real benefit here is that scopes are chainable so they can be inserted into an overall "query pipeline" where you can easily extend things. In my case I had a site listing habits and I realized that habits needed a state of active (if it was true it should be displayed and if it was false then it shouldn't be). Here was my initial view code:

<%plan.habits.each do |habit|%>
  <li><%=link_to(habit.name, habit_path(habit))%></li>
<% end %>

Once I had the scope defined then all I had to do was this:

<%plan.habits.active.each do |habit|%>
  <li><%=link_to(habit.name, habit_path(habit))%></li>
<% end %>

And blammo I had an active list of habits.

Now let's carry the example further and let's say that you want a list of habits sorted by name. You can define a scope called ordered_by_name like this:

scope :order_by_name, -> { order("name ASC")}

and the scope into your view like this:

<%plan.habits.active.order_by_name.each do |habit|%>
  <li><%=link_to(habit.name, habit_path(habit))%></li>
<% end %>

And it will be sorted by name, easy peasy. Now I've used scopes for years (prior to their current syntax actually) but their elegance has never struck me in quite the same way. Perhaps it is just Monday …

More on Rails Scopes: