1 files changed, 71 insertions, 0 deletions
diff --git a/spec/lib/gitlab/database/loose_index_scan_distinct_count_spec.rb b/spec/lib/gitlab/database/loose_index_scan_distinct_count_spec.rb
new file mode 100644
index 00000000000..e0eac26e4d9
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+++ b/spec/lib/gitlab/database/loose_index_scan_distinct_count_spec.rb
@@ -0,0 +1,71 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Database::LooseIndexScanDistinctCount do
+  context 'counting distinct users' do
+    let_it_be(:user) { create(:user) }
+    let_it_be(:other_user) { create(:user) }
+
+    let(:column) { :creator_id }
+
+    before_all do
+      create_list(:project, 3, creator: user)
+      create_list(:project, 1, creator: other_user)
+    end
+
+    subject(:count) { described_class.new(Project, :creator_id).count(from: Project.minimum(:creator_id), to: Project.maximum(:creator_id) + 1) }
+
+    it { is_expected.to eq(2) }
+
+    context 'when STI model is queried' do
+      it 'does not raise error' do
+        expect { described_class.new(Group, :owner_id).count(from: 0, to: 1) }.not_to raise_error
+      end
+    end
+
+    context 'when model with default_scope is queried' do
+      it 'does not raise error' do
+        expect { described_class.new(GroupMember, :id).count(from: 0, to: 1) }.not_to raise_error
+      end
+    end
+
+    context 'when the fully qualified column is given' do
+      let(:column) { 'projects.creator_id' }
+
+      it { is_expected.to eq(2) }
+    end
+
+    context 'when AR attribute is given' do
+      let(:column) { Project.arel_table[:creator_id] }
+
+      it { is_expected.to eq(2) }
+    end
+
+    context 'when invalid value is given for the column' do
+      let(:column) { Class.new }
+
+      it { expect { described_class.new(Group, column) }.to raise_error(Gitlab::Database::LooseIndexScanDistinctCount::ColumnConfigurationError) }
+    end
+
+    context 'when null values are present' do
+      before do
+        create_list(:project, 2).each { |p| p.update_column(:creator_id, nil) }
+      end
+
+      it { is_expected.to eq(2) }
+    end
+  end
+
+  context 'counting STI models' do
+    let!(:groups) { create_list(:group, 3) }
+    let!(:namespaces) { create_list(:namespace, 2) }
+
+    let(:max_id) { Namespace.maximum(:id) + 1 }
+
+    it 'counts groups' do
+      count = described_class.new(Group, :id).count(from: 0, to: max_id)
+      expect(count).to eq(3)
+    end
+  end
+end