Gestionnaire: add follow(dossier) method

app/models/gestionnaire.rb

@@ -45,6 +45,12 @@ class Gestionnaire < ActiveRecord::Base
     nil
   end
 
+  def follow(dossier)
+    return if follow?(dossier)
+
+    followed_dossiers << dossier
+  end
+
   def follow?(dossier)
     followed_dossiers.include?(dossier)
   end

spec/models/gestionnaire_spec.rb

@@ -73,6 +73,25 @@ describe Gestionnaire, type: :model do
     end
   end
 
+  describe 'follow' do
+    let(:dossier) { create :dossier }
+    let(:already_followed_dossier) { create :dossier }
+
+    before { gestionnaire.followed_dossiers << already_followed_dossier }
+
+    context 'when a gestionnaire follow a dossier for the first time' do
+      before { gestionnaire.follow(dossier) }
+
+      it { expect(gestionnaire.follow?(dossier)).to be true }
+    end
+
+    context 'when a gestionnaire follows a dossier already followed' do
+      before { gestionnaire.follow(already_followed_dossier) }
+
+      it { expect(gestionnaire.follow?(already_followed_dossier)).to be true }
+    end
+  end
+
   describe '#follow?' do
     let!(:dossier) { create :dossier, procedure: procedure }