Oliver Oliver - 3 months ago 12
Ruby Question

How do I force rails to not use a cached result for has_many through relations?

I have the following three models (massively simplified):

class A < ActiveRecord::Base
has_many :bs
has_many :cs, :through => :bs

class B < ActiveRecord::Base
belongs_to :a
has_many :cs

class C < ActiveRecord::Base
belongs_to :b

It seems that A.cs gets cached the first time it is used (per object) when I'd really rather it not.

Here's a console session that highlights the problem (the fluff has been edited out)

First, the way it should work

rails console
001 > b = B.create
002 > c = C.new
003 > c.b = b
004 > c.save
005 > a = A.create
006 > a.bs << b
007 > a.cs
=> [#<C id: 1, b_id: 1>]

This is indeed as you would expect. The a.cs is going nicely through the a.bs relation.

And now for the caching infuriations

008 > a2 = A.create
009 > a2.cs
=> []
010 > a2.bs << b
011 > a2.cs
=> []

So the first call to a2.cs (resulting in a db query) quite correctly returned no Cs. The second call, however, shows a distinct lack of Cs even though they jolly well should be there (no db queries occurred).

And just to test my sanity is not to blame

012 > A.find(a2.id).cs
=> [#<C id: 1, b_id: 1>]

Again, a db query was performed to get both the A record and the associated C's.

So, back to the question: How do I force rails to not use the cached result? I could of course resign myself to doing this workaround (as shown in console step 12), but since that would result in an extra two queries when only one is necessary, I'd rather not.


I did some more research into this issue. While using clear_association_cache was convenient enough, adding it after every operation that invalidated the cache did not feel DRY. I thought Rails should be able to keep track of this. Thankfully, there is a way!

I will use your example models: A (has many B, has many C through B), B (belongs to A, has many C), and C (belongs to B).

We will need to use the touch: true option for the belongs_to method. This method updates the updated_at attribute on the parent model, but more importantly it also triggers an after_touch callback. This callback allows to us to automatically clear the association cache for any instance of A whenever a related instance of B or C is modified, created, or destroyed.

First modify the belongs_to method calls for B and C, adding touch:true

class B < ActiveRecord::Base
  belongs_to :a, touch: true
  has_many   :cs

class C < ActiveRecord::Base
  belongs_to :b, touch: true

Then add an after_touch callback to A

class A < ActiveRecord::Base
  has_many :bs
  has_many :cs, through: :bs

  after_touch :clear_association_cache

Now we can safely hack away, creating all sorts of methods that modify/create/destroy instances of B and C, and the instance of A that they belong to will automatically have its cache up to date without us having to remember to call clear_association_cache all over the place.

Depending on how you use model B, you may want to add an after_touch callback there as well.

Documentation for belongs_to options and ActiveRecord callbacks:


Hope this helps!