The Ugly Truth
On a recent project, we had an ActiveRecord model that declared some relationships and callbacks like so:
belongs_to :credit_card before_create :build_credit_card
The intent was that
build_credit_card would build the associated
CreditCard instance, and ActiveRecord’s default
:autosave feature on the
belongs_to would save it.
What we discovered was that no
CreditCard object was being persisted. We confirmed that
:autosave is on by default for
belongs_to relationships, so we couldn’t immediately understand why the new
CreditCard wasn’t being created.
Googling proved futile, so we dove right in to the ActiveRecord source- and boy did we have a good laugh about 10 minutes later.
What we found was that the
:autosave option works by simply declaring a
before_save callback- that makes perfect sense.
In our case, however, we were building the object to be autosaved in a
before_create callback, which ActiveRecords runs after the
before_save callbacks (cf. the callback ordering docs).
So our first problem was that we needed to move the call to
build_credit_card from a
before_create callback to a
before_save :on => :create callback.
Did you catch that? There is a difference between
before_save :on => :create. A big difference.
While I understand the how and why of this, the semantics don’t make it obvious. So beware!
Now with our declarations changed to
belongs_to :credit_card before_save :build_credit_card, :on => :create
We ran our tests again, and, still, no love. Ahhh, we’ve still got an ordering problem. In addition to the ordering semantics detailed in the docs, ActiveRecord also runs callbacks within a single group in the order in which they are declared. So, even though we changed the call to
build_credit_card to occur in a
before_save, it was still occurring after the
before_save callback, because of the declaration order.
Finally, we changed our declarations to
before_save :build_credit_card, :on => :create belongs_to :credit_card
and our tests were happy.
autosavewith any ActiveRecord association, be very careful of callback ordering if you are building or modifying the inverse objects using ActiveRecord callbacks.
before_createisn’t ever the same thing as
before_save :on => :create, even if it sounds like it should be.
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