ActiveRecord为与has_and_belongs_to_many的MySQL关系生成昂贵的DESCRIBE查询

Question

We have a Rails3 app we've just started development on. We use a has_and_belongs_to_many relation between two models. Every time we use one of these models, ActiveRecord executes a DESCRIBE-query on the join-table, eg:

SQL (1.1ms)  describe `articles_tags`

These queries are relatively expensive (that is, more than 10 times slower than the actual SELECT-query) and are executed very often. They don't seem necessary to me: the table was automatically generated because of the has_and_belongs_to_many-relation so ActiveRecord should already be aware of its structure (just an article_id and a tag_id column).

Is there a good reason for this behaviour? If not, how do I stop it from happening?

Answer 1

In development mode, the server constantly recreates all the classes so you don't have to restart the server for every change. In production, your models will be cached and it shouldn't need to describe the tables constantly. In your development.rb file, you'll see this line:

config.cache_classes = false

Which drives this behavior. It should be set to true in production.rb

Answer 2

If your table doesn't have a primary key it will run describe on each query. I fixed mine adding a primary key. Works even in dev mode.