Add Category list method

Add a method to return a flat list of categories for a given root. This
will be useful in places as this replicates to functionality of the
older `PublicBodyCategory` model.

Fixes #8081

app/helpers/public_body_helper.rb

@@ -37,7 +37,7 @@ def public_body_not_requestable_reasons(public_body)
   #
   # Returns a String
   def type_of_authority(public_body)
-    categories = PublicBody.categories.
+    categories = PublicBody.category_list.
       where(category_tag: public_body.tag_string.split).order(:id)
 
     types = categories.each_with_index.map do |category, index|

app/models/category.rb

@@ -53,6 +53,27 @@ def tree
     children.includes(:translations, children: [:translations])
   end
 
+  def list
+    sql = <<~SQL.squish
+      WITH RECURSIVE nested_categories AS (
+        SELECT child_id
+        FROM category_relationships
+        WHERE parent_id = :parent_id
+
+        UNION ALL
+
+        SELECT cr.child_id
+        FROM category_relationships cr
+        INNER JOIN nested_categories nc ON cr.parent_id = nc.child_id
+      )
+      SELECT DISTINCT c.id FROM categories c
+      INNER JOIN nested_categories nc ON c.id = nc.child_id;
+    SQL
+
+    ids = Category.find_by_sql([sql, { parent_id: id }]).map(&:id)
+    Category.where(id: ids).includes(:translations)
+  end
+
   private
 
   def check_tag_assignments

app/models/concerns/categorisable.rb

@@ -18,5 +18,9 @@ def self.category_root
     def self.categories
       category_root.tree
     end
+
+    def self.category_list
+      category_root.list
+    end
   end
 end

spec/helpers/public_body_helper_spec.rb

@@ -82,10 +82,11 @@
       expect(type_of_authority(public_body)).to eq(expected)
     end
 
-    context 'when associated with one category' do
+    context 'when categories are descendents of non-root parents' do
       it 'returns the description wrapped in an anchor tag' do
         FactoryBot.create(
-          :category, :public_body,
+          :category,
+          parents: [FactoryBot.build(:category, :public_body)],
           category_tag: 'spec',
           description: 'spec category'
         )
@@ -94,13 +95,12 @@
         anchor = %Q(<a href="/body/list/spec">Spec category</a>)
         expect(type_of_authority(public_body)).to eq(anchor)
       end
-    end
 
-    context 'when associated with several categories' do
       it 'joins the category descriptions and capitalizes the first letter' do
         3.times do |i|
           FactoryBot.create(
-            :category, :public_body,
+            :category,
+            parents: [FactoryBot.build(:category, :public_body)],
             category_tag: "spec_#{i}",
             description: "spec category #{i}"
           )

spec/models/category_spec.rb

@@ -130,6 +130,14 @@
       FactoryBot.create(:category, title: 'Branch', parents: [trunk])
     end
 
+    let!(:other_root) do
+      FactoryBot.create(:category, title: 'InfoRequest')
+    end
+
+    let!(:other_trunk) do
+      FactoryBot.create(:category, title: 'Trunk', parents: [other_root])
+    end
+
     around do |example|
       @query_count = 0
       subscription = ActiveSupport::Notifications.subscribe(
@@ -141,7 +149,7 @@
       ActiveSupport::Notifications.unsubscribe(subscription)
     end
 
-    it 'returns root category descendents' do
+    it 'returns root category descendents as tree' do
       expect(root.tree).to match_array([trunk])
       expect(root.tree[0].children).to match_array([branch])
     end
@@ -159,4 +167,51 @@
       }.to_not change { @query_count }
     end
   end
+
+  describe '#list' do
+    subject { root.list }
+
+    let!(:root) do
+      FactoryBot.create(:category, title: 'PublicBody')
+    end
+
+    let!(:trunk) do
+      FactoryBot.create(:category, title: 'Trunk', parents: [root])
+    end
+
+    let!(:branch) do
+      FactoryBot.create(:category, title: 'Branch', parents: [trunk])
+    end
+
+    let!(:other_root) do
+      FactoryBot.create(:category, title: 'InfoRequest')
+    end
+
+    let!(:other_trunk) do
+      FactoryBot.create(:category, title: 'Trunk', parents: [other_root])
+    end
+
+    around do |example|
+      @query_count = 0
+      subscription = ActiveSupport::Notifications.subscribe(
+        'sql.active_record'
+      ) { @query_count += 1 }
+
+      example.call
+
+      ActiveSupport::Notifications.unsubscribe(subscription)
+    end
+
+    it 'returns root category descendents as list' do
+      expect(root.list).to match_array([trunk, branch])
+    end
+
+    it 'preload translations' do
+      # load list and perform all necessary DB queries
+      list = root.list.to_a
+
+      # iterate through list and ensure translations have been preloaded
+      expect { list.each(&:title) }.to_not change { @query_count }
+    end
+  end
 end

spec/models/concerns/categorisable.rb

@@ -13,4 +13,11 @@
       described_class.categories
     end
   end
+
+  describe '.category_list' do
+    it 'calls category_root.list' do
+      expect(described_class).to receive_message_chain(:category_root, :list)
+      described_class.category_list
+    end
+  end
 end