It's a way of keeping the record count of an association in the parent
table.
So if you had:
posts
- id
- title
- body
- comment_count
Comments
- id
- body
- post_id
Then the comment count field for a post would contain the number of
posts. This means that if you did something like
Post p = postsRepository.Find(3);
Console.WriteLine(p.Count);
You wouldn't hit the database because the count on the association
would be loaded from the posts table.
In AR there are hooks on the associations that increment and decrement
the counter cache if it's available.
Does that make sense?
Here's some more from AR's documentation:
# :counter_cache - caches the number of belonging objects on the
associate class through the use of increment_counter and
decrement_counter. The counter cache is incremented when an object of
this class is created and decremented when it‘s destroyed. This
requires that a column named #{table_name}_count (such as
comments_count for a belonging Comment class) is used on the associate
class (such as a Post class). You can also specify a custom counter
cache column by providing a column name instead of a true/false value
to this option (e.g., :counter_cache => :my_custom_counter.) Note:
Specifying a counter_cache will add it to that model‘s list of
readonly attributes using attr_readonly.
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