1 files changed, 60 insertions, 0 deletions
diff --git a/spec/lib/gitlab/pagination/keyset/in_operator_optimization/strategies/record_loader_strategy_spec.rb b/spec/lib/gitlab/pagination/keyset/in_operator_optimization/strategies/record_loader_strategy_spec.rb
new file mode 100644
index 00000000000..5180403b493
--- /dev/null
+++ b/spec/lib/gitlab/pagination/keyset/in_operator_optimization/strategies/record_loader_strategy_spec.rb
@@ -0,0 +1,60 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Pagination::Keyset::InOperatorOptimization::Strategies::RecordLoaderStrategy do
+  let(:finder_query) { -> (created_at_value, id_value) { Project.where(Project.arel_table[:id].eq(id_value)) } }
+  let(:model) { Project }
+
+  let(:keyset_scope) do
+    scope, _ = Gitlab::Pagination::Keyset::SimpleOrderBuilder.build(
+      Project.order(:created_at, :id)
+    )
+
+    scope
+  end
+
+  let(:keyset_order) do
+    Gitlab::Pagination::Keyset::Order.extract_keyset_order_object(keyset_scope)
+  end
+
+  let(:order_by_columns) do
+    Gitlab::Pagination::Keyset::InOperatorOptimization::OrderByColumns.new(keyset_order.column_definitions, model.arel_table)
+  end
+
+  subject(:strategy) { described_class.new(finder_query, model, order_by_columns) }
+
+  describe '#initializer_columns' do
+    # Explanation:
+    # > SELECT NULL::projects AS records
+    #
+    # The query returns one row and one column. The column may contain a full project row.
+    # In this particular case the row is NULL.
+    it 'returns a NULL table row as the result column' do
+      expect(strategy.initializer_columns).to eq(["NULL::projects AS records"])
+    end
+  end
+
+  describe '#columns' do
+    # Explanation:
+    # > SELECT (SELECT projects FROM projects limit 1)
+    #
+    # Selects one row from the database and collapses it into one column.
+    #
+    # Side note: Due to the type casts, columns and initializer_columns can be also UNION-ed:
+    # SELECT * FROM (
+    #   (
+    #     SELECT NULL::projects AS records
+    #     UNION
+    #     SELECT (SELECT projects FROM projects limit 1)
+    #   )
+    # ) as records
+    it 'uses the finder query to load the row in the result column' do
+      expected_loader_query = <<~SQL
+        (SELECT projects FROM "projects" WHERE "projects"."id" = recursive_keyset_cte.projects_id_array[position] LIMIT 1)
+      SQL
+
+      expect(strategy.columns).to eq([expected_loader_query.chomp])
+    end
+  end
+end