Why does adding a bang make create work? - ruby-on-rails

Question

Rails convention of adding a bang to the end of a method makes it throw an exception if it fails. But my experience isn't matching that behavior.
My controller for a many-to-many relationship (an audit trail). The relationship object cannot be created, only updated by posting events to the audit trail object. (Which means you create by updating...)
I have a User object and a Foo object I'll call the relationship Bar.
bar=Bar.where(:user_id=>params[:user_id]).where(:foo_id=>params[:foo_id]).first
if bar
authorize! :update, bar
else
user=User.find(params[:user_id])
authorize! :bar_create, user
foo=Foo.find(params[:foo_id])
bar=Bar.create!(:user_id=>user.id, :foo_id=>foo.id)
end
The create method does not work. I debugged, and bar.save worked fine, but the entire point of create is to avoid having to make that second call to save. I experimented, and discovered that create! works just fine.
Edit:
As I continued on, I discovered that create! did not, in fact, always save. No errors in the underlying object, just mysteriously not saved.
I've had to do a create call followed by a save call, which... honestly, I just don't understand.
Edit: Per request, adding model code -- simplified to the relevant statements by removing unnecessary methods, validation calls, and the like. (While writing this, I noticed that I haven't yet added the has_many :through calls, but... doesn't seem like those should be relevant to the issue at hand.
class User < ActiveRecord::Base
has_secure_password
has_many :progresses
end
class Bar < ActiveRecord::Base
belongs_to :user
belongs_to :foo
has_many :bar_events
validates :user, :presence=>true
validates :foo, :presence=>true
scope :user_id, -> (user_id){ where user_id: user_id}
scope :foo_id, -> (foo_id){ where foo_id: foo_id}
end
class Foo < ActiveRecord::Base
end

Answer 1

There is a validation error or more in one of the associations in Bar foo or user. Try inspecting those objects:
bar=Bar.create!(:user_id=>user.id, :foo_id=>foo.id)
puts bar.errors.inspect, bar.user.errors.inspect, bar.foo.errors.inspect
That will print the errors of all those objects to the terminal running rails server. The only reason create would not save is due to validation errors in itself or nested associations.