1 files changed, 208 insertions, 0 deletions
diff --git a/spec/graphql/resolvers/concerns/caching_array_resolver_spec.rb b/spec/graphql/resolvers/concerns/caching_array_resolver_spec.rb
new file mode 100644
index 00000000000..b6fe94a2312
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+++ b/spec/graphql/resolvers/concerns/caching_array_resolver_spec.rb
@@ -0,0 +1,208 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe ::CachingArrayResolver do
+  include GraphqlHelpers
+
+  let_it_be(:non_admins) { create_list(:user, 4, admin: false) }
+  let(:query_context) { {} }
+  let(:max_page_size) { 10 }
+  let(:field) { double('Field', max_page_size: max_page_size) }
+  let(:schema) { double('Schema', default_max_page_size: 3) }
+
+  let_it_be(:caching_resolver) do
+    mod = described_class
+
+    Class.new(::Resolvers::BaseResolver) do
+      include mod
+
+      def query_input(is_admin:)
+        is_admin
+      end
+
+      def query_for(is_admin)
+        if is_admin.nil?
+          model_class.all
+        else
+          model_class.where(admin: is_admin)
+        end
+      end
+
+      def model_class
+        User # Happens to include FromUnion, and is cheap-ish to create
+      end
+    end
+  end
+
+  describe '#resolve' do
+    context 'there are more than MAX_UNION_SIZE queries' do
+      let_it_be(:max_union) { 3 }
+      let_it_be(:resolver) do
+        mod = described_class
+        max = max_union
+
+        Class.new(::Resolvers::BaseResolver) do
+          include mod
+
+          def query_input(username:)
+            username
+          end
+
+          def query_for(username)
+            if username.nil?
+              model_class.all
+            else
+              model_class.where(username: username)
+            end
+          end
+
+          def model_class
+            User # Happens to include FromUnion, and is cheap-ish to create
+          end
+
+          define_method :max_union_size do
+            max
+          end
+        end
+      end
+
+      it 'executes the queries in multiple batches' do
+        users = create_list(:user, (max_union * 2) + 1)
+        expect(User).to receive(:from_union).twice.and_call_original
+
+        results = users.in_groups_of(2, false).map do |users|
+          resolve(resolver, args: { username: users.map(&:username) }, field: field, schema: schema)
+        end
+
+        expect(results.flat_map(&method(:force))).to match_array(users)
+      end
+    end
+
+    context 'all queries return results' do
+      let_it_be(:admins) { create_list(:admin, 3) }
+
+      it 'batches the queries' do
+        expect do
+          [resolve_users(true), resolve_users(false)].each(&method(:force))
+        end.to issue_same_number_of_queries_as { force(resolve_users(nil)) }
+      end
+
+      it 'finds the correct values' do
+        found_admins = resolve_users(true)
+        found_others = resolve_users(false)
+        admins_again = resolve_users(true)
+        found_all = resolve_users(nil)
+
+        expect(force(found_admins)).to match_array(admins)
+        expect(force(found_others)).to match_array(non_admins)
+        expect(force(admins_again)).to match_array(admins)
+        expect(force(found_all)).to match_array(admins + non_admins)
+      end
+    end
+
+    it 'does not perform a union of a query with itself' do
+      expect(User).to receive(:where).once.and_call_original
+
+      [resolve_users(false), resolve_users(false)].each(&method(:force))
+    end
+
+    context 'one of the queries returns no results' do
+      it 'finds the correct values' do
+        found_admins = resolve_users(true)
+        found_others = resolve_users(false)
+        found_all = resolve_users(nil)
+
+        expect(force(found_admins)).to be_empty
+        expect(force(found_others)).to match_array(non_admins)
+        expect(force(found_all)).to match_array(non_admins)
+      end
+    end
+
+    context 'one of the queries has already been cached' do
+      before do
+        force(resolve_users(nil))
+      end
+
+      it 'avoids further queries' do
+        expect do
+          repeated_find = resolve_users(nil)
+
+          expect(force(repeated_find)).to match_array(non_admins)
+        end.not_to exceed_query_limit(0)
+      end
+    end
+
+    context 'the resolver overrides item_found' do
+      let_it_be(:admins) { create_list(:admin, 2) }
+      let(:query_context) do
+        {
+          found: { true => [], false => [], nil => [] }
+        }
+      end
+
+      let_it_be(:with_item_found) do
+        Class.new(caching_resolver) do
+          def item_found(key, item)
+            context[:found][key] << item
+          end
+        end
+      end
+
+      it 'receives item_found for each key the item mapped to' do
+        found_admins = resolve_users(true, with_item_found)
+        found_all = resolve_users(nil, with_item_found)
+
+        [found_admins, found_all].each(&method(:force))
+
+        expect(query_context[:found]).to match({
+          false => be_empty,
+          true => match_array(admins),
+          nil => match_array(admins + non_admins)
+        })
+      end
+    end
+
+    context 'the max_page_size is lower than the total result size' do
+      let(:max_page_size) { 2 }
+
+      it 'respects the max_page_size, on a per subset basis' do
+        found_all = resolve_users(nil)
+        found_others = resolve_users(false)
+
+        expect(force(found_all).size).to eq(2)
+        expect(force(found_others).size).to eq(2)
+      end
+    end
+
+    context 'the field does not declare max_page_size' do
+      let(:max_page_size) { nil }
+
+      it 'takes the page size from schema.default_max_page_size' do
+        found_all = resolve_users(nil)
+        found_others = resolve_users(false)
+
+        expect(force(found_all).size).to eq(schema.default_max_page_size)
+        expect(force(found_others).size).to eq(schema.default_max_page_size)
+      end
+    end
+
+    specify 'force . resolve === to_a . query_for . query_input' do
+      r = resolver_instance(caching_resolver)
+      args = { is_admin: false }
+
+      naive = r.query_for(r.query_input(**args)).to_a
+
+      expect(force(r.resolve(**args))).to eq(naive)
+    end
+  end
+
+  def resolve_users(is_admin, resolver = caching_resolver)
+    args = { is_admin: is_admin }
+    resolve(resolver, args: args, field: field, ctx: query_context, schema: schema)
+  end
+
+  def force(lazy)
+    ::Gitlab::Graphql::Lazy.force(lazy)
+  end
+end