3 files changed, 25 insertions, 4 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index d14b5b27f8..4bc31457c0 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   `find_in_batches` now returns an `Enumerator` that can calculate its size.
+
+    See also #13938.
+
+    *Marc-André Lafortune*
+
 *   Make sure transaction state gets reset after a commit operation on the record.
 
     If a new transaction was open inside a callback, the record was loosing track
diff --git a/activerecord/lib/active_record/relation/batches.rb b/activerecord/lib/active_record/relation/batches.rb
index 666cef80a9..a06da659cb 100644
--- a/activerecord/lib/active_record/relation/batches.rb
+++ b/activerecord/lib/active_record/relation/batches.rb
@@ -96,17 +96,22 @@ module ActiveRecord
     # the batch sizes.
     def find_in_batches(options = {})
       options.assert_valid_keys(:start, :batch_size)
-      return to_enum(:find_in_batches, options) unless block_given?
 
       relation = self
+      start = options[:start]
+      batch_size = options[:batch_size] || 1000
+
+      unless block_given?
+        return to_enum(:find_in_batches, options) do
+          total = start ? where(table[primary_key].gteq(start)).size : size
+          (total - 1).div(batch_size) + 1
+        end
+      end
 
       if logger && (arel.orders.present? || arel.taken.present?)
         logger.warn("Scoped order and limit are ignored, it's forced to be batch order and batch size")
       end
 
-      start = options[:start]
-      batch_size = options[:batch_size] || 1000
-
       relation = relation.reorder(batch_order).limit(batch_size)
       records = start ? relation.where(table[primary_key].gteq(start)).to_a : relation.to_a
 
diff --git a/activerecord/test/cases/batches_test.rb b/activerecord/test/cases/batches_test.rb
index 8216d74cb3..4ee09a640e 100644
--- a/activerecord/test/cases/batches_test.rb
+++ b/activerecord/test/cases/batches_test.rb
@@ -191,4 +191,14 @@ class EachTest < ActiveRecord::TestCase
       end
     end
   end
+
+  if Enumerator.method_defined? :size
+    def test_find_in_batches_should_return_a_sized_enumerator
+      assert_equal 11, Post.find_in_batches(:batch_size => 1).size
+      assert_equal 6, Post.find_in_batches(:batch_size => 2).size
+      assert_equal 4, Post.find_in_batches(:batch_size => 2, :start => 4).size
+      assert_equal 4, Post.find_in_batches(:batch_size => 3).size
+      assert_equal 1, Post.find_in_batches(:batch_size => 10_000).size
+    end
+  end
 end