Model.reflections gives information about a model’s associations. It is a Hash keyed on the association name. e.g.
Post.reflections.keys # => ["comments"]
Here is an example of some of the information it can be used to access:
Post.reflections["comments"].table_name # => "comments"
Post.reflections["comments"].macro # => :has_many
Post.reflections["comments"].foreign_key # => "message_id"
Note: this answer has been updated to cover Rails 4.2 based on MCB’s answer and the comments below. In earlier versions of Rails the reflection’s foreign_key was accessed using primary_key_name instead, and the keys for the reflections may be symbols instead of strings depending on how the association was defined e.g. :comments instead of "comments".