You will have to define that the model is embedding an office object and vice versa, explanation here: http://mongoid.org/en/mongoid/docs/relations.html. I'm guessing that you need a 1-N relation, so that a Professional can embed several offices? In that case, something like this should work.
Professional model
class Professional
include Mongoid::Document
field :first_name, type: String
field :last_name, type: String
field :company_name, type: String
field :address, type: String
validates :first_name, length: { minimum: 5, :message => "What" }, format: { with: /\A[a-zA-Z]+\z/, message: "only allows letters" }
embeds_many :offices, class_name: "Office"
end
Office model
class Office
include Mongoid::Document
field :name, type: String
field :address, type: String
embedded_in :professional, :inverse_of => :offices
end
Remember that if you are going to use one form for these objects you'll have to do a nested form, something like (or just google something up):
<%= form_for @professional, :url => { :action => "create" } do |o| %>
<%= o.text_field :first_name %>
<%= o.text_field :last_name %>
<%= o.fields_for :office do |builder| %>
<%= builder.text_field :name %>
<%= builder.text_field :address %>
<% end %>
<% end %>
Note that nothing is tested.