Return a Placements::SupportUser for suppor users

Make sure the `current_user` method returns an instance of
Placements::SupportUser when in the Placements service and the current
user has `support_user = true`.

While `Placements::User` and `Placements::SupportUser` are both
effectively the same right now, we envisage functional changes in the
near future. For example, `user.organisations` will differ to
`support_user.organisations`.

app/models/dfe_sign_in_user.rb

@@ -47,7 +47,16 @@ def self.load_from_session(session)
 
   def user
     # TODO: When dfe sign-in is fully implemented, we will be able to find the user by the id == id_token.
-    @user ||= user_klass.find_by(email:)
+    @user ||=
+      begin
+        user = user_klass.find_by(email:)
+
+        if service == :placements && user&.support_user?
+          user = user.becomes(Placements::SupportUser)
+        end
+
+        user
+      end
   end
 
   def self.end_session!(session)

spec/models/dfe_sign_in_user_spec.rb

@@ -48,7 +48,7 @@
 
   describe "#user" do
     describe "claims service" do
-      it "returns the claims user" do
+      it "returns the current Claims::User" do
         claims_user = create(:claims_user)
 
         session = {
@@ -63,11 +63,12 @@
         dfe_sign_in_user = DfESignInUser.load_from_session(session)
 
         expect(dfe_sign_in_user.user).to eq claims_user
+        expect(dfe_sign_in_user.user).to be_a Claims::User
       end
     end
 
     describe "placements service" do
-      it "returns the placements user" do
+      it "returns the current Placements::User" do
         placements_user = create(:placements_user)
 
         session = {
@@ -82,6 +83,25 @@
         dfe_sign_in_user = DfESignInUser.load_from_session(session)
 
         expect(dfe_sign_in_user.user).to eq placements_user
+        expect(dfe_sign_in_user.user).to be_a Placements::User
+      end
+
+      it "returns a Placements::SupportUser for support users" do
+        support_user = create(:placements_user, :support)
+
+        session = {
+          "dfe_sign_in_user" => {
+            "first_name" => support_user.first_name,
+            "last_name" => support_user.last_name,
+            "email" => support_user.email
+          },
+          "service" => :placements
+        }
+
+        dfe_sign_in_user = DfESignInUser.load_from_session(session)
+
+        expect(dfe_sign_in_user.user.id).to eq support_user.id
+        expect(dfe_sign_in_user.user).to be_a Placements::SupportUser
       end
     end
   end