Collapse where constraints to one where constraint

In order to remove duplication with joining arel where constraints with
`AND`, all constraints on `build_arel` are collapsed into one head node: `Arel::Nodes::And`

Closes: #11963

activerecord/CHANGELOG.md

+*   Fix: joins association, with defined in the scope block constraints by using several
+    where constraints and at least of them is not `Arel::Nodes::Equality`,
+    generates invalid SQL expression.
+
+    Fixes: #11963
+
+    *Paul Nikitochkin*
+
 *   Re-use `order` argument pre-processing for `reorder`.
 
     *Paul Nikitochkin*

activerecord/lib/active_record/relation/query_methods.rb

@@ -882,14 +882,13 @@ def custom_join_ast(table, joins)
     end
 
     def collapse_wheres(arel, wheres)
-      equalities = wheres.grep(Arel::Nodes::Equality)
-
-      arel.where(Arel::Nodes::And.new(equalities)) unless equalities.empty?
-
-      (wheres - equalities).each do |where|
+      predicates = wheres.map do |where|
+        next where if ::Arel::Nodes::Equality === where
         where = Arel.sql(where) if String === where
-        arel.where(Arel::Nodes::Grouping.new(where))
+        Arel::Nodes::Grouping.new(where)
       end
+
+      arel.where(Arel::Nodes::And.new(predicates)) if predicates.present?
     end
 
     def build_where(opts, other = [])

activerecord/test/cases/associations/inner_join_association_test.rb

@@ -41,6 +41,11 @@ def test_join_conditions_added_to_join_clause
     assert_no_match(/WHERE/i, sql)
   end
 
+  def test_join_association_conditions_support_string_and_arel_expressions
+    assert_equal 0, Author.joins(:welcome_posts_with_comment).count
+    assert_equal 1, Author.joins(:welcome_posts_with_comments).count
+  end
+
   def test_join_conditions_allow_nil_associations
     authors = Author.includes(:essays).where(:essays => {:id => nil})
     assert_equal 2, authors.count

activerecord/test/models/author.rb

@@ -31,6 +31,13 @@ def ratings
   has_many :thinking_posts, -> { where(:title => 'So I was thinking') }, :dependent => :delete_all, :class_name => 'Post'
   has_many :welcome_posts, -> { where(:title => 'Welcome to the weblog') }, :class_name => 'Post'
 
+  has_many :welcome_posts_with_comment,
+           -> { where(title: 'Welcome to the weblog').where('comments_count = ?', 1) },
+           class_name: 'Post'
+  has_many :welcome_posts_with_comments,
+           -> { where(title: 'Welcome to the weblog').where(Post.arel_table[:comments_count].gt(0)) },
+           class_name: 'Post'
+
   has_many :comments_desc, -> { order('comments.id DESC') }, :through => :posts, :source => :comments
   has_many :limited_comments, -> { limit(1) }, :through => :posts, :source => :comments
   has_many :funky_comments, :through => :posts, :source => :comments