Get field constraints (handle model constraints)

[Sewake]

def validate_unique(self, values):
    '''
    Perform a check on possible existing objects 
    if the field has a unique constraint
    
    Return True if validated, else False
    '''
    values = [val for val in values if val]
    unique_values = set(values)
    # Check if all values are unique
    if len(values) != len(unique_values):
        return False
    # Check if values are not already in database
    lookups = [
        models.Q(**{self.field: val})
        for val in unique_values
        if val
    ]
    lookup = reduce(operator.or_, lookups)
    return not self.model.objects.filter(lookup).exists()

widgets.ForeignKeyWidget.validate_unique is not implemented yet because we need to know if the field linked with the widget has a unique constraint or not.
If this is the case, we run validate_unique to prevent import when clauses are not respected (avoid IntegrityError exceptions).