问题
I'm getting 'rake aborted! ... posts_count is marked readonly' errors.
I have two models: user and post.
users has_many posts.
posts belongs_to :user, :counter_cache => true
I have a migration which adds the posts_count column to the users table and then calculates and records the current number of posts per user.
self.up
add_column :users, :posts_count, :integer, :default => 0
User.reset_column_information
User.all.each do |u|
u.update_attribute( :posts_count, u.posts.count)
end
end
when I run the migration I get the error. This is pretty clear-cut, of course and if I remove the :counter_cache declaration from the posts model, e.g.
belongs_to :user
the migration runs fine. This obviously, does not make sense because you couldn't really implement it this way. What am I missing?
回答1:
You should be using User.reset_counters to do this. Additionally, I would recommend using find_each instead of each because it will iterate the collection in batches instead of all at once.
self.up
add_column :users, :posts_count, :integer, :default => 0
User.reset_column_information
User.find_each do |u|
User.reset_counters u.id, :posts
end
end
回答2:
OK, the documentation states:
Counter cache columns are added to the containing model’s list of read-only attributes through attr_readonly.
I think this is what happens: you declare the counter in the model's definition, thus rendering the "posts_count" attribute read-only. Then, in the migration, you attempt to update it directly, resulting in the error you mention.
The quick-and-dirty solution is to remove the counter_cache declaration from the model, run the migration (in order to add the required column to the database AND populate it with the current post counts), and then re-add the counter_cache declaration to the model. Should work but is nasty and requires manual intervention during the migration - not a good idea.
I found this blog post which suggests altering the model's list of read-only attributes during the migration, it's a bit oudated but you might want to give it a try.
来源：https://stackoverflow.com/questions/4295404/problem-with-counter-cache-implementation