id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
5514	Consistency issue when using ForeignKeys	Philippe Raoult	Philippe Raoult	"Consider the following models:
{{{
class Operand(models.Model):
    value_1 = models.CharField(maxlength=200)
    value_2 = models.CharField(maxlength=200)

class Operation(models.Model):
    arg = models.ForeignKey(Operand)
}}}

Now let's suppose we want to do some work on Operands:
{{{
>>> common_arg = Operand(value_1=""val1"", value_2=""val2"")
>>> common_arg.save()
>>> op1 = Operation(arg = common_arg)
>>> op2 = Operation(arg = common_arg)
>>> op1.save()
>>> op2.save()
>>> operations = Operation.objects.all().select_related()
>>> operations
[<Operation: Operation object>, <Operation: Operation object>]
>>> operations[0].arg.value_1 = ""new_val1""
>>> operations[0].arg.save()
>>> operations[1].arg.value_2 = ""new_val2""
>>> operations[1].arg.save()
>>> modified_arg = Operand.objects.get(id=1)
# OOPS we lost one change here !!
>>> modified_arg.value_1
u'val1'
>>> modified_arg.value_2
u'new_val2'

# restore the initial state 
>>> modified_arg.value_2 = ""val2""
>>> modified_arg.save()

# this is not limited to select_related but pervasive with FKs
>>> work_to_do = [(op, op.arg) for op in Operation.objects.all()]
>>> work_to_do[0][1].value_1 = ""new_val1""
>>> work_to_do[0][1].value_1
'new_val1'

# oops, operation 2 will use the outdated argument
>>> work_to_do[1][1].value_1
u'val1'
}}}

#17 could provide a solution for this issue: caching instances in memory so get() and friends can return the instance we already have instead of creating a new, distinct one. 

The issue is especially easy to encounter with select_related because all related fields will be preloaded in distinct instances in memory. Walking Operation.objects.all() and 
modifying the Operands from there would load an new instance from the DB for each operation and work as expected in most cases. "		closed	Core (Other)	dev		wontfix	select_related		Design decision needed	0	0	0	0	0	0