Add helper queries class for application urn

FindApplicationIndexQuery will be used to retrieve the index of an
created application and to later match it with a random urn

app/queries/applications_index_query.rb

@@ -0,0 +1,33 @@
+class ApplicationsIndexQuery
+  delegate :to_sql, to: :query
+
+  def execute
+    ApplicationRecord.connection.execute(to_sql)
+  end
+
+  def query
+    @query ||= applications
+                 .project(projection)
+                 .order(applications[:created_at].asc)
+  end
+
+  def projection
+    [
+      applications[:id],
+      row_number.as("application_index"),
+    ]
+  end
+
+  def row_number
+    Arel::Nodes::Over.new(
+      Arel::Nodes::NamedFunction.new("ROW_NUMBER", []),
+      Arel.sql("(ORDER BY created_at ASC)"),
+    )
+  end
+
+private
+
+  def applications
+    Application.arel_table
+  end
+end

app/queries/find_application_index_query.rb

@@ -0,0 +1,37 @@
+class FindApplicationIndexQuery
+  delegate :to_sql, to: :query
+  attr_reader :application_id
+
+  def initialize(application_id:)
+    @application_id = application_id
+  end
+
+  def execute
+    ApplicationRecord.connection.execute(to_sql)
+  end
+
+  def query
+    @query ||= manager
+                 .project(projection)
+                 .from(from_clause)
+                 .where(where_clause)
+  end
+
+  def projection
+    [Arel.star]
+  end
+
+  def from_clause
+    ApplicationsIndexQuery.new.query.as("list")
+  end
+
+  def where_clause
+    Arel.sql("list.id").eq(application_id)
+  end
+
+private
+
+  def manager
+    @manager ||= Arel::SelectManager.new(Arel::Table.engine)
+  end
+end

spec/queries/applications_index_query_spec.rb

@@ -0,0 +1,12 @@
+require "rails_helper"
+
+RSpec.describe ApplicationsIndexQuery, type: :model do
+  subject(:sql) { described_class.new.to_sql }
+
+  describe "#to_sql" do
+    it { expect(sql).to include("SELECT") }
+    it { expect(sql).to include('"applications"."id"') }
+    it { expect(sql).to include("ROW_NUMBER() OVER (ORDER BY created_at ASC) AS application_index") }
+    it { expect(sql).to include('FROM "applications"') }
+  end
+end

spec/queries/find_application_index_query_spec.rb

@@ -0,0 +1,14 @@
+require "rails_helper"
+
+RSpec.describe FindApplicationIndexQuery, type: :model do
+  subject(:sql) { described_class.new(application_id: application.id).to_sql }
+
+  let(:application) { create(:application) }
+  let(:subquery) { ApplicationsIndexQuery.new.to_sql }
+
+  describe "#to_sql" do
+    it { expect(sql).to include("SELECT *") }
+    it { expect(sql).to include("FROM (#{subquery}) list") }
+    it { expect(sql).to include("WHERE list.id = #{application.id}") }
+  end
+end