How to invalidate has_many relationship in Rails

Question

In my Rails app, I have a class with a has_many relationship. For the sake of efficiency, I want to do some direct SQL to update many rows in the database at once, and then I want to mark the has_many relationship as no longer valid. If later code accesses the has_many relationship, I want it to reload the data. But I obviously want to skip the SQL unless it is necessary.

So for example:

class Student
  has_many courses # may have a :condition clause or some such

  def some_method
    # For some reason, we want to change all courses in here, not
    # just a single row.
    ActiveRecord::Base.connection.execute("UPDATE courses SET location = #{new_location}")
    # Not sure if we'll later do anything with self.courses, so I need to invalidate
    # that relationship.  Could do self.courses.reload right here, but I don't want to
    # do the SQL if it isn't necessary; the cache only lasts until the end of the
    # current page request.
  end
end

I may well be missing something rather obvious. Some hypothetical self.courses.invalidate method.

Answer 1

Expansion of what I couldn't say in 140 characters... You can avoid needing to invalidate relations by using some of Rails' convenience methods that are attached to an association. Let's say, for example, that I have a User and a Project model:

class Project < ActiveRecord::Base
  # Backing table has :id, :user_id, :name, and :description columns
end

class User < ActiveRecord::Base
  has_many :projects
end

user = User.first # Assuming you have some users already

# Creates a new project named "Some Project" with description "Some Description"
# and sets the project's user_id to user.id. Also handles in-memory user.projects
# updating.
user.projects.create(:name => "Some Project, :description => "Some Description")

Answer 2

You should have a look at the build in update_attribute methods which update via SQL but have the advantage of changing the local attributes.

Answer 3

Not in their public API, but you could try the reset method on the AssociationCollection class.

Watching the log:

s = Student.first
s.courses       # Hits db
s.courses       # Hits cache 
s.courses.reset # No db
s.courses       # Hits db