1 files changed, 75 insertions, 0 deletions
diff --git a/spec/models/concerns/safely_change_column_default_spec.rb b/spec/models/concerns/safely_change_column_default_spec.rb
new file mode 100644
index 00000000000..36782170eaf
--- /dev/null
+++ b/spec/models/concerns/safely_change_column_default_spec.rb
@@ -0,0 +1,75 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe SafelyChangeColumnDefault, feature_category: :database do
+  include Gitlab::Database::DynamicModelHelpers
+  before do
+    ApplicationRecord.connection.execute(<<~SQL)
+      CREATE TABLE _test_gitlab_main_data(
+        id bigserial primary key not null,
+        value bigint default 1
+      );
+    SQL
+  end
+
+  let!(:model) do
+    define_batchable_model('_test_gitlab_main_data', connection: ApplicationRecord.connection).tap do |model|
+      model.include(described_class)
+      model.columns_changing_default(:value)
+      model.columns # Force the schema cache to populate
+    end
+  end
+
+  def alter_default(new_default)
+    ApplicationRecord.connection.execute(<<~SQL)
+      ALTER TABLE _test_gitlab_main_data ALTER COLUMN value SET DEFAULT #{new_default}
+    SQL
+  end
+
+  def recorded_insert_queries(&block)
+    recorder = ActiveRecord::QueryRecorder.new
+    recorder.record(&block)
+
+    recorder.log.select { |q| q.include?('INSERT INTO') }
+  end
+
+  def query_includes_value_column?(query)
+    parsed = PgQuery.parse(query)
+    parsed.tree.stmts.first.stmt.insert_stmt.cols.any? { |node| node.res_target.name == 'value' }
+  end
+
+  it 'forces the column to be written on a change' do
+    queries = recorded_insert_queries do
+      model.create!(value: 1)
+    end
+
+    expect(queries.length).to eq(1)
+
+    expect(query_includes_value_column?(queries.first)).to be_truthy
+  end
+
+  it 'does not write the column without a change' do
+    queries = recorded_insert_queries do
+      model.create!
+    end
+
+    expect(queries.length).to eq(1)
+    expect(query_includes_value_column?(queries.first)).to be_falsey
+  end
+
+  it 'does not send the old column value if the default has changed' do
+    alter_default(2)
+    model.create!
+
+    expect(model.pluck(:value)).to contain_exactly(2)
+  end
+
+  it 'prevents writing new default in place of the old default' do
+    alter_default(2)
+
+    model.create!(value: 1)
+
+    expect(model.pluck(:value)).to contain_exactly(1)
+  end
+end