diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 3c44660b3a..a37f1ad540 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Fixed polymorphic eager loading when using a String as foreign key.
+
+    Fixes #14734.
+
+    *Lauro Caetano*
+
 *   Changed scoped blocks to be executed with `instance_eval`
 
     Named scopes (i.e. using STI) were previously cached according to
diff --git a/activerecord/lib/active_record/associations/preloader/association.rb b/activerecord/lib/active_record/associations/preloader/association.rb
index bf461070e0..5a172a417d 100644
--- a/activerecord/lib/active_record/associations/preloader/association.rb
+++ b/activerecord/lib/active_record/associations/preloader/association.rb
@@ -58,7 +58,11 @@ module ActiveRecord
 
         def owners_by_key
           @owners_by_key ||= owners.group_by do |owner|
-            owner[owner_key_name]
+            if owner_key_type == association_key_type
+              owner[owner_key_name]
+            else
+              owner[owner_key_name].to_s
+            end
           end
         end
 
@@ -93,13 +97,24 @@ module ActiveRecord
           records_by_owner
         end
 
+        def association_key_type
+          @klass.column_types[association_key_name.to_s].type
+        end
+
+        def owner_key_type
+          @model.column_types[owner_key_name.to_s].type
+        end
+
         def load_slices(slices)
           @preloaded_records = slices.flat_map { |slice|
             records_for(slice)
           }
 
           @preloaded_records.map { |record|
-            [record, record[association_key_name]]
+            key = record[association_key_name]
+            key = key.to_s unless owner_key_type == association_key_type
+
+            [record, key]
           }
         end
 
diff --git a/activerecord/test/cases/associations/belongs_to_associations_test.rb b/activerecord/test/cases/associations/belongs_to_associations_test.rb
index dc7314b450..9c92dc1141 100644
--- a/activerecord/test/cases/associations/belongs_to_associations_test.rb
+++ b/activerecord/test/cases/associations/belongs_to_associations_test.rb
@@ -570,6 +570,19 @@ class BelongsToAssociationsTest < ActiveRecord::TestCase
     assert companies(:first_client).readonly_firm.readonly?
   end
 
+  def test_test_polymorphic_assignment_foreign_key_type_string
+    comment = Comment.first
+    comment.author   = Author.first
+    comment.resource = Member.first
+    comment.save
+
+    assert_equal Comment.all.to_a,
+      Comment.includes(:author).to_a
+
+    assert_equal Comment.all.to_a,
+      Comment.includes(:resource).to_a
+  end
+
   def test_polymorphic_assignment_foreign_type_field_updating
     # should update when assigning a saved record
     sponsor = Sponsor.new
diff --git a/activerecord/test/models/comment.rb b/activerecord/test/models/comment.rb
index ea261ac9ec..3cb32d9346 100644
--- a/activerecord/test/models/comment.rb
+++ b/activerecord/test/models/comment.rb
@@ -7,6 +7,9 @@ class Comment < ActiveRecord::Base
   scope :created, -> { all }
 
   belongs_to :post, :counter_cache => true
+  belongs_to :author,   polymorphic: true
+  belongs_to :resource, polymorphic: true
+
   has_many :ratings
 
   belongs_to :first_post, :foreign_key => :post_id
diff --git a/activerecord/test/schema/schema.rb b/activerecord/test/schema/schema.rb
index 9e511af3bd..67ba358843 100644
--- a/activerecord/test/schema/schema.rb
+++ b/activerecord/test/schema/schema.rb
@@ -187,6 +187,9 @@ ActiveRecord::Schema.define do
     t.integer :taggings_count, default: 0
     t.integer :children_count, default: 0
     t.integer :parent_id
+    t.references :author, polymorphic: true
+    t.string :resource_id
+    t.string :resource_type
   end
 
   create_table :companies, force: true do |t|