Don't clear sequence name when we explicitly assign it.

activerecord/lib/active_record/model_schema.rb

@@ -123,7 +123,7 @@ def table_name=(value)
         @table_name          = value
         @quoted_table_name   = nil
         @arel_table          = nil
-        @sequence_name       = nil
+        @sequence_name       = nil unless defined?(@explicitly_sequence_name) && @explicitly_sequence_name
         @relation            = Relation.new(self, arel_table)
       end
 
@@ -170,7 +170,8 @@ def sequence_name
       end
 
       def reset_sequence_name #:nodoc:
-        self.sequence_name = connection.default_sequence_name(table_name, primary_key)
+        @sequence_name = connection.default_sequence_name(table_name, primary_key)
+        @explicitly_sequence_name = false
       end
 
       # Sets the name of the sequence to use when generating ids to the given
@@ -189,6 +190,7 @@ def reset_sequence_name #:nodoc:
       #   end
       def sequence_name=(value)
         @sequence_name = value.to_s
+        @explicitly_sequence_name = true
       end
 
       # Indicates whether the table associated with this class exists

activerecord/test/cases/base_test.rb

@@ -1492,6 +1492,17 @@ def test_clear_cash_when_setting_table_name
     assert_not_equal before_seq, after_seq unless before_seq.blank? && after_seq.blank?
   end
 
+  def test_dont_clear_sequence_name_when_setting_explicitly
+    Joke.sequence_name = "black_jokes_seq"
+    Joke.table_name    = "cold_jokes"
+    before_seq         = Joke.sequence_name
+
+    Joke.table_name    = "funny_jokes"
+    after_seq          = Joke.sequence_name
+
+    assert_equal before_seq, after_seq unless before_seq.blank? && after_seq.blank?
+  end
+
   def test_set_table_name_symbol_converted_to_string
     Joke.table_name = :cold_jokes
     assert_equal 'cold_jokes', Joke.table_name