How to get activerecord associations via reflection

For normal columns, you can get at them via the columns class method. However, associations may be named something quite different if the foreign_key option is set in the relationship method. For example, given

class Post
  has_many :comments, :foreign_key => :message_id # this is a contrived example
end

if I did Post.column_names I could get at message_id, but is there any way to get comments?

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".

For future Googlers in Rails 4 the answer would now be:

Post.reflections[:comments].foreign_key # => "message_id"

Taken from here: https://stackoverflow.com/a/15364743/2167965

EDIT:

reflections, as of 4.2, now takes strings instead of symbols which is a fun bug to track down. If you want to keep using symbols you should switch to reflect_on_association(:assoc_name). Also note reflections are actually the public api which will keep reporting things like HABTM, even though it's all has many through under the hood. The reflections Rails is actually using are now in _reflections