Find inverse associations with plural names

Previously ActiveRecord couldn't find inverse associations if they were plural, which is a pretty standard use case. This commit changes the behavior to first attempt to find the singular version, then attempt to find the plural version. That makes it work and find plural associations as in the example below:

```
class Post
  has_many :comments
end

class Comment
  belongs_to :post
end
```

Previously the `:post` association reflection would only attempt to find a `comment` inverse, as opposed to both a `comment` and `comments` inverse.

2 files changed, 26 insertions, 5 deletions

diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index 6d2f75a3ae..b2110f727c 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -612,9 +612,21 @@ module ActiveRecord
 
         # returns either +nil+ or the inverse association name that it finds.
         def automatic_inverse_of
-          if can_find_inverse_of_automatically?(self)
-            inverse_name = ActiveSupport::Inflector.underscore(options[:as] || active_record.name.demodulize).to_sym
+          return unless can_find_inverse_of_automatically?(self)
 
+          inverse_name_candidates =
+            if options[:as]
+              [options[:as]]
+            else
+              active_record_name = active_record.name.demodulize
+              [active_record_name, ActiveSupport::Inflector.pluralize(active_record_name)]
+            end
+
+          inverse_name_candidates.map! do |candidate|
+            ActiveSupport::Inflector.underscore(candidate).to_sym
+          end
+
+          inverse_name_candidates.detect do |inverse_name|
             begin
               reflection = klass._reflect_on_association(inverse_name)
             rescue NameError
@@ -623,9 +635,7 @@ module ActiveRecord
               reflection = false
             end
 
-            if valid_inverse_reflection?(reflection)
-              return inverse_name
-            end
+            valid_inverse_reflection?(reflection)
           end
         end
 
diff --git a/activerecord/test/cases/associations/inverse_associations_test.rb b/activerecord/test/cases/associations/inverse_associations_test.rb
index da3a42e2b5..0808aa7a52 100644
--- a/activerecord/test/cases/associations/inverse_associations_test.rb
+++ b/activerecord/test/cases/associations/inverse_associations_test.rb
@@ -20,6 +20,7 @@ require "models/company"
 require "models/project"
 require "models/author"
 require "models/post"
+require "models/department"
 
 class AutomaticInverseFindingTests < ActiveRecord::TestCase
   fixtures :ratings, :comments, :cars
@@ -724,6 +725,16 @@ class InversePolymorphicBelongsToTests < ActiveRecord::TestCase
     # fails because Interest does have the correct inverse_of
     assert_raise(ActiveRecord::InverseOfAssociationNotFoundError) { Face.first.polymorphic_man = Interest.first }
   end
+
+  def test_favors_has_one_associations_for_inverse_of
+    inverse_name = Post.reflect_on_association(:author).inverse_of.name
+    assert_equal :post, inverse_name
+  end
+
+  def test_finds_inverse_of_for_plural_associations
+    inverse_name = Department.reflect_on_association(:hotel).inverse_of.name
+    assert_equal :departments, inverse_name
+  end
 end
 
 # NOTE - these tests might not be meaningful, ripped as they were from the parental_control plugin