Make filter_binds filter out symbols that are equal to strings

ActiveRecord::Relation::Merger's filter_binds method does not filter out bind
variables when one of the attribute nodes has a string name, but the other has
a symbol name, even when those names are actually equal.

This can result in there being more bind variables than placeholders in the
generated SQL.  This is particularly an issue for PostgreSQL, where this is
treated as an error.

This patch changes the filter_binds method to make it convert both attribute
names to strings before comparing.

activerecord/CHANGELOG.md

+*   `ActiveRecord::Relation::Merger#filter_binds` now compares equivalent symbols and 
+    strings in column names as equal.
+    
+    This fixes a rare case in which more bind values are passed than there are
+    placeholders for them in the generated SQL statement, which can make PostgreSQL
+    throw a `StatementInvalid` exception.
+    
+    *Nat Budin*
+
 *   `change_column_default` allows `[]` as argument to `change_column_default`.
 
     Fixes #11586.

activerecord/lib/active_record/relation/merger.rb

@@ -156,7 +156,7 @@ def merge_single_values
       def filter_binds(lhs_binds, removed_wheres)
         return lhs_binds if removed_wheres.empty?
 
-        set = Set.new removed_wheres.map { |x| x.left.name }
+        set = Set.new removed_wheres.map { |x| x.left.name.to_s }
         lhs_binds.dup.delete_if { |col,_| set.include? col.name }
       end
 

activerecord/test/cases/relation/merging_test.rb

@@ -108,6 +108,11 @@ def test_merging_reorders_bind_params
     merged = left.merge(right)
     assert_equal post, merged.first
   end
+  
+  def test_merging_compares_symbols_and_strings_as_equal
+    post = PostThatLoadsCommentsInAnAfterSaveHook.create!(title: "First Post", body: "Blah blah blah.")
+    assert_equal "First comment!", post.comments.where(body: "First comment!").first_or_create.body
+  end
 end
 
 class MergingDifferentRelationsTest < ActiveRecord::TestCase

activerecord/test/models/comment.rb

@@ -40,3 +40,11 @@ class SubSpecialComment < SpecialComment
 
 class VerySpecialComment < Comment
 end
+
+class CommentThatAutomaticallyAltersPostBody < Comment
+  belongs_to :post, class_name: "PostThatLoadsCommentsInAnAfterSaveHook", foreign_key: :post_id
+  
+  after_save do |comment|
+    comment.post.update_attributes(body: "Automatically altered")
+  end
+end
\ No newline at end of file

activerecord/test/models/post.rb

@@ -208,3 +208,12 @@ class SpecialPostWithDefaultScope < ActiveRecord::Base
   self.table_name = 'posts'
   default_scope { where(:id => [1, 5,6]) }
 end
+
+class PostThatLoadsCommentsInAnAfterSaveHook < ActiveRecord::Base
+  self.table_name = 'posts'
+  has_many :comments, class_name: "CommentThatAutomaticallyAltersPostBody", foreign_key: :post_id
+  
+  after_save do |post|
+    post.comments.load
+  end
+end