models: require belong_to associations on champ

- Make `champ.dossier` a requirement;
- Move the dossier_id assignation to `before_validation` (otherwise
the record is invalid, and never gets saved);
- Allow specs to only build the champ (instead of saving it to the
database), which bypasses the requirement to have a dossier.

spec/controllers/champs/siret_controller_spec.rb

@@ -1,14 +1,14 @@
 describe Champs::SiretController, type: :controller do
   let(:user) { create(:user) }
-  let(:procedure) { create(:procedure, :published) }
+  let(:procedure) do
+    tdc_siret = build(:type_de_champ_siret, procedure: nil)
+    create(:procedure, :published, types_de_champ: [tdc_siret])
+  end
 
   describe '#show' do
     let(:dossier) { create(:dossier, user: user, procedure: procedure) }
-    let(:champ) do
-      d = dossier
-      type_de_champ = create(:type_de_champ_siret, procedure: procedure)
-      type_de_champ.champ.create(dossier: d, value: nil, etablissement: nil)
-    end
+    let(:champ) { dossier.champs.first }
+
     let(:params) do
       {
         champ_id: champ.id,
@@ -27,6 +27,7 @@ describe Champs::SiretController, type: :controller do
       let(:api_etablissement_status) { 200 }
       let(:api_etablissement_body) { File.read('spec/fixtures/files/api_entreprise/etablissements.json') }
       let(:token_expired) { false }
+
       before do
         sign_in user
         stub_request(:get, /https:\/\/entreprise.api.gouv.fr\/v2\/etablissements\/#{siret}?.*token=/)
@@ -112,7 +113,7 @@ describe Champs::SiretController, type: :controller do
           champ.reload
           expect(champ.value).to eq(siret)
           expect(champ.etablissement.siret).to eq(siret)
-          expect(champ.reload.etablissement.naf).to eq("6202A")
+          expect(champ.etablissement.naf).to eq("6202A")
           expect(dossier.reload.etablissement).to eq(nil)
         end
       end