Cast field value upon assignment

[warvariuc]

When you assign a value to a model instance field, the value is not immediately converted to the type the field should hold:

ipdb> thread.__class__.tutor_id
<mongoengine.base.fields.ObjectIdField object at 0x7f30443c3f50>
ipdb> thread.tutor_id = '558143a3bb16fb194ff98526'
ipdb> thread.tutor_id
'558143a3bb16fb194ff98526'
ipdb> thread.student_id
ObjectId('558143a3bb16fb194ff98522')

This leads sometimes to bugs which are difficult to discover. In the case above thread.tutor_id was assigned an id which came from a POST request. Then later in the code I am using thread.tutor_id as in a query filter, which fails, because 558143a3bb16fb194ff98526 is not the same as ObjectId('558143a3bb16fb194ff98526').

[MRigal]

Valid point, even if it may make the system slower... I don't what is best, add a keyword argument to enable the feature on a per-field basis or maybe simply change the comparison method...

[warvariuc]

I don't think it'll make system slower -- the validation/conversion happens only once. But failing fast is a good approach.