How to get activerecord associations via reflection

Question

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

Answer 1

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"

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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".

Answer 2

For an ActiveRecord object I use:

object._reflections

So, I can manipulate the Hash returned. For instance:

object._reflections.keys.each do |key|
    object.public_send(key).destroy_all
end

The above example delete all the relationships from database.

Answer 3

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