On Dec 19, 2011, at 8:13 PM, Jackson, Cameron wrote:
> A have a Machines table and a Parts table. A machine is made up of many
> parts, and each part could be in many machines, so clearly this is a M:M
> relationship. I've implemented this with a secondary table, which has two
> columns: machine_id and part_id. There is no primary key or unique constraint
> on the secondary table, just two foreign keys.
>
> So far so good.
>
> The problem is when I want to have 2 of a part in a machine. It was no
> problem doing this:
>
> machine = Machine('theMachine')
> machine.parts.append(session.query(Part).filter_by(name =
> 'thePart').first())
> machine.parts.append(session.query(Part).filter_by(name =
> 'thePart').first())
> session.add(machine)
>
> I now have two identical rows in the secondary table, which is what I want.
> However, when I try to retrieve the object:
>
> print session.query(Machine).filter_by(name = 'theMachine').first().parts
>
> I just get:
>
> InstrumentedList: [<Part(u'thePart')>]
>
> The duplicates are being cut out, which is very annoying. This Stack Overflow
> question seems relevant, but I wasn't to glean any useful information from it.
>
> How can I have duplicate entries in my M:M relationship?
many-to-many assumes <left_id>/<right_id> are unique. Those cols should be a
primary key, or there should otherwise be another column on the table that's
primary, which would turn it into an association.
The fact that you have no other information stored with each
<left_id>/<right_id>, but you'd like two of them, violates normal form.
Actually first normal form. (background,
http://en.wikipedia.org/wiki/First_normal_form )
At minimum, the additional information you might want to associate with each
association between machine and part is how many of them are present, meaning
there would be just one row between part and machine. Or perhaps a simple
index (a counter for each one). But in relational databases, all rows should
express something unique that isn't anywhere else.
>
> Cheers,
> Cam
>
> Cameron Jackson
> Engineering Intern
> Air Operations
> Thales Australia
> Thales Australia Centre, WTC Northbank Wharf, Concourse Level,
> Siddeley Street, Melbourne, VIC 3005, Australia
> Tel: +61 3 8630 4591
> cameron.jack...@thalesgroup.com.au | www.thalesgroup.com.au
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