This is related to this post
<http://stackoverflow.com/questions/32825381/handle-multiple-relations-between-two-rows-objects-in-sqlalchemy>
I found this problem in my application using Python3 with a SQLAlchemy
connected sqlite-database.
Simple description:
Entity `a` (from table/class `A`) can have multible references to
entity `b` (from table/class `B`).
Sample:
sqlite> SELECT oid FROM A;
1
sqlite> SELECT oid, val FROM B;
1|0
2|1
sqlite> SELECT a_oid, b_oid FROM a_b_relation;
1|1
1|1
You see here that `a` has two references to the same entity/row `b`.
In my data structure this make sense. But maybe it break a
SQL-/RDBMS-rule?
When I do this in Python3 with the object it causes an error because
SQLAlchemy try to execute an DELETE statement for that.
a._bbb.clear()
The error
DELETE FROM a_b_relation WHERE a_b_relation.a_oid = ? AND
a_b_relation.b_oid = ? (1, 1)
ROLLBACK
sqlalchemy.orm.exc.StaleDataError: DELETE statement on table
'a_b_relation' expected to delete 1 row(s); Only 2 were matched.
The error make sense for me in that case. But I don't know how I could
handle that.
Maybe it goes a bit to far but I will describe the original
data-structure, too. It is about making usage statistic for a gym. :)
You walk to a machine and lift some kilograms a couple of times - that
is called `TrainingUnit` (what is equivalent to `A` in the example).
For warming up you make 12 repetitions and lift up 35 kg in each of
them - that is called a `SetSet` (equivalent to `B` in the example;
keep in mind that `Set` is a reserved SQL-keyword). Then you make a
one-minute-break and do 12 repetitions with 40 kg - the second set. And
you do a third again(!) with 12 repetitions and 40 kg again. So the
`TrainingUnit` instance/row need three relations to instances/rows of
`SetSet`. Because the second and third set have the same
values/settings I would use the same instance/row of `SetSet` here.
Most of the time the sportsman does TrainingUnits each day in the same
configuration - which means each day the sets have the same values.
That is why I want to reuse that instances/rows of `SetSet`. And the
number of sets per TrainingUnit are not fixed - it depends on the
sportsman how many sets he/she will make.
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