Mongoid Model won't persist has_one via embeds_one relation - ruby-on-rails-4

Question

I'm having trouble with embeds in a mongoid4-based rails 4 app. I've been looking for an answer everywhere for the past 2 days. So here is the code.
This is a church management app, with a Service model, embedding a team and a band. Each team/band has several roles such as "presidence", "communion" that refer to a user.
My models :
class Service
include Mongoid::Document
...
embeds_one :team, autobuild: true
embeds_one :band, autobuild: true
...
accepts_nested_attributes_for :team, :band
end
class Team
include Mongoid::Document
embedded_in :service
has_one :presidence, :class_name => 'User', autosave: true
has_one :message, :class_name => 'User', autosave: true
...
end
class Band
include Mongoid::Document
has_one :lead, :class_name => 'User', autosave: true
has_one :guitar, :class_name => 'User', autosave: true
...
embedded_in :service
end
class User
include Mongoid::Document
embeds_one :profile
belongs_to :team, :inverse_of => :presidence
belongs_to :team, :inverse_of => :message
belongs_to :band, :inverse_of => :lead
belongs_to :band, :inverse_of => :guitar
def fullname
"#{profile.firstname} #{profile.lastname}"
end
def self.find_by_talent(talent)
self.where("profile.talents.name" => talent)
end
end
The services controller :
# POST /services
# POST /services.json
def create
#service = Service.new(service_params)
respond_to do |format|
if #service.save
format.html { redirect_to #service, notice: 'Service was successfully created.' }
format.json { render action: 'show', status: :created, location: #service }
else
format.html { render action: 'new' }
format.json { render json: #service.errors, status: :unprocessable_entity }
end
end
end
...
def service_params
params.require(:service).permit(:date, :time, :place, :name, :theme, :team => [:id, :precedence, :message ], :band => [:id, :lead, :guitar ])
end
And the form in _form.html.erb :
<%= form_for(#service) do |f| %>
...
<%= f.fields_for #service.team do |tf| %>
<%= tf.collection_select :presidence, User.find_by_talent(:presidence), :_id, :fullname, {:include_blank => "select a person"} %>
<%= tf.collection_select :message, User.find_by_talent(:message), :id, :fullname, {:include_blank => "select a person"} %>
<% end %>
<%= f.fields_for #service.band do |bf| %>
<%= bf.collection_select :lead, User.find_by_talent(:lead), :id, :fullname, {:include_blank => "select a person"} %>
<%= bf.collection_select :guitar, User.find_by_talent(:guitar), :id, :fullname, {:include_blank => "select a person"} %>
<% end %>
...
<% end %>
When creating a service, everything seems to run fine, but this is what I get in the console :
2.0.0-p195 :001 > s = Service.last
=> #<Service _id: 52ea18834d61631e7e020000, date: "2014-02-02", time: "10:00", place: "Where it's at", name: "My great name", theme: "The service's theme">
2.0.0-p195 :002 > s.team
=> #<Team _id: 52ea18834d61631e7e030000, >
2.0.0-p195 :003 > s.team.presidence
=> nil
Why is s.team.presidence not created ? s.team looks weird, too, with an ending comma...
Here is the content of my rails log :
Started POST "/services" for 127.0.0.1 at 2014-01-30 10:16:51 +0100
Processing by ServicesController#create as HTML
Parameters: {"utf8"=>"✓", "authenticity_token"=>"Ph6lbdHC2FbiANn/fGSzHWprenj3fWKXM40Hrsc5+AM=", "service"=>{"date"=>"2014-02-02", "name"=>"My great name", "theme"=>"The service's theme", "time"=>"10:00", "place"=>"Where it's at", "team"=>{"presidence"=>"52ea18324d61631e81010000", "message"=>"52ea18324d61631e81010000"}, "band"=>{"lead"=>"52ea18324d61631e81010000", "guitar"=>"52ea18324d61631e81010000"}}, "commit"=>"Create Service"}
MOPED: 127.0.0.1:27017 COMMAND database=admin command={:ismaster=>1} runtime: 0.6610ms
MOPED: 127.0.0.1:27017 UPDATE database=service_boot_camp_development collection=users selector={"band_id"=>BSON::ObjectId('52ea18834d61631e7e010000'), "_id"=>{"$nin"=>[]}} update={"$set"=>{"band_id"=>nil}} flags=[:multi]
COMMAND database=service_boot_camp_development command={:getlasterror=>1, :w=>1} runtime: 0.5800ms
MOPED: 127.0.0.1:27017 INSERT database=service_boot_camp_development collection=services documents=[{"_id"=>BSON::ObjectId('52ea18834d61631e7e020000'), "date"=>"2014-02-02", "time"=>"10:00", "place"=>"Where it's at", "name"=>"My great name", "theme"=>"The service's theme", "team"=>{"_id"=>BSON::ObjectId('52ea18834d61631e7e030000')}, "band"=>{"_id"=>BSON::ObjectId('52ea18834d61631e7e010000')}}] flags=[]
COMMAND database=service_boot_camp_development command={:getlasterror=>1, :w=>1} runtime: 2.7460ms
I guess I'm doing something wrong, but I have no clue if it is in the database model or in the form... or anything else...

Answer 1

You will not be able to do it this way. When you create an embedded document, its _id and all of its data are embedded directly within the parent document. This is in contrast to an association, where the document with the belongs_to gets a foreign key which points to its associated parent document. So here, your User documents each have a team_id and band_id, but when the database tries to get the documents, it can't find them, since you can't query directly for embedded documents; you need the parent document first. For more, see the Mongoid documentation.
Another potential issue is that you have multiple belongs_to definitions in the User models. This will also cause an issue, because for each one of those, Mongoid will attempt to create a team_id and band_id. You should name them separately and specify a class name; perhaps names like :presiding_team and :message_team, :lead_band and :guitar_band, etc. This answer should show you what that would look like.
I would recommend making the Team and Band separate referenced documents instead of embedded documents, since you won't be able to reference users effectively while they're embedded.
Hope this helps.