Posted by flyerhzm on June 15, 2013
activerecord provides default_scope to set a default scope for all operations on the model, it looks convenient at first, but will lead to some unexpected behaviors, we should avoid using it.
I have used default_scope many times before, but later I regretted doing so.
Assume we defined a default_scope in Post model
class Post default_scope where(published: true).order("created_at desc") end
default_scope added some behaviors that you may not expect.
1. You can't override default scope. e.g. by default, it lists posts order by created_at,
> Post.limit(10) Post Load (3.3ms) SELECT `posts`.* FROM `posts` WHERE `posts`.`published` = 1 ORDER BY created_at desc LIMIT 10
if you want to display posts order by updated_at rather than created_at, you may use the following line,
> Post.order("updated_at desc").limit(10) Post Load (17.3ms) SELECT `posts`.* FROM `posts` WHERE `posts`.`published` = 1 ORDER BY created_at desc, updated_at desc LIMIT 10
but as you can see, it order by both created_at and updated_at, default scope is not overriden, you have to use unscoped to disable default scope explicitly,
> Post.unscoped.order("updated_at desc").limit(10) Post Load (1.9ms) SELECT `posts`.* FROM `posts` ORDER BY updated_at desc LIMIT 10
so you have to remember the model has a default_scope, and add unscoped if you want to override the default scope, it's an accident waiting to happen.
2. default_scope will affect your model initialization. e.g.
> Post.new => #<Post id: nil, title: nil, created_at: nil, updated_at: nil, user_id: nil, published: true>
most developers are not aware this point, they think default_scope only affect queries, it is the wrong way default_scope does.
Don't use default_scope any more, just define it as a scope and explictly call that scope.