Add Joins relation parsers

lib/gitlab/usage/metrics/names_suggestions/relation_parsers/joins.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Usage
+    module Metrics
+      module NamesSuggestions
+        module RelationParsers
+          class Joins < ::Arel::Visitors::PostgreSQL
+            def accept(object)
+              object.source.right.map do |join|
+                visit(join, collector)
+              end
+            end
+
+            private
+
+            # rubocop:disable Naming/MethodName
+            def visit_Arel_Nodes_StringJoin(object, collector)
+              result = visit(object.left, collector)
+              source, constraints = result.value.split('ON')
+              {
+                source: source.split('JOIN').last&.strip,
+                constraints: constraints&.strip
+              }.compact
+            end
+
+            def visit_Arel_Nodes_FullOuterJoin(object, _)
+              parse_join(object)
+            end
+
+            def visit_Arel_Nodes_OuterJoin(object, _)
+              parse_join(object)
+            end
+
+            def visit_Arel_Nodes_RightOuterJoin(object, _)
+              parse_join(object)
+            end
+
+            def visit_Arel_Nodes_InnerJoin(object, _)
+              {
+                source: visit(object.left, collector).value,
+                constraints: object.right ? visit(object.right.expr, collector).value : nil
+              }.compact
+            end
+            # rubocop:enable Naming/MethodName
+
+            def parse_join(object)
+              {
+                source: visit(object.left, collector).value,
+                constraints: visit(object.right.expr, collector).value
+              }
+            end
+
+            def quote(value)
+              "#{value}"
+            end
+
+            def quote_table_name(name)
+              "#{name}"
+            end
+
+            def quote_column_name(name)
+              "#{name}"
+            end
+
+            def collector
+              Arel::Collectors::SubstituteBinds.new(@connection, Arel::Collectors::SQLString.new)
+            end
+          end
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/usage/metrics/names_suggestions/relation_parsers/joins_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Usage::Metrics::NamesSuggestions::RelationParsers::Joins do
+  describe '#accept' do
+    let(:collector) { Arel::Collectors::SubstituteBinds.new(ActiveRecord::Base.connection, Arel::Collectors::SQLString.new) }
+
+    context 'with join added via string' do
+      it 'collects join parts' do
+        arel = Issue.joins('LEFT JOIN projects ON projects.id = issue.project_id')
+
+        arel = arel.arel
+        result = described_class.new(ApplicationRecord.connection).accept(arel)
+
+        expect(result).to match_array [{ source: "projects", constraints: "projects.id = issue.project_id" }]
+      end
+    end
+
+    context 'with join added via arel node' do
+      it 'collects join parts' do
+        source_table = Arel::Table.new('records')
+        joined_table = Arel::Table.new('joins')
+        second_level_joined_table = Arel::Table.new('second_level_joins')
+
+        arel = source_table
+                 .from
+                 .project(source_table['id'].count)
+                 .join(joined_table, Arel::Nodes::OuterJoin)
+                 .on(source_table[:id].eq(joined_table[:records_id]))
+                 .join(second_level_joined_table, Arel::Nodes::OuterJoin)
+                 .on(joined_table[:id].eq(second_level_joined_table[:joins_id]))
+
+        result = described_class.new(ApplicationRecord.connection).accept(arel)
+
+        expect(result).to match_array [{ source: "joins", constraints: "records.id = joins.records_id" }, { source: "second_level_joins", constraints: "joins.id = second_level_joins.joins_id" }]
+      end
+    end
+  end
+end