M2M Question
Matthew Pava
I have a Document model that has an M2M-symmetrical field on itself. The idea is that we can “link” documents to other documents. It’s very simple to find the linked documents of a particular document, but is there an easy way to get all the linked documents of the linked documents of the document in question? I will probably have to write my own QuerySet or ModelManager function, but I was just curious if Django had something built in.
For example:
A is linked to J, C, K.
è J is linked to A
è C is linked to A
è K is linked to A
B is linked to J, M, P.
è J is linked to B
è M is linked to B
è P is linked to B
C is linked to O.
è O is linked to C
D is linked to O, P, R.
è O is linked to D
è P is linked to D
è R is linked to D
So when I get the entire “chain” of documents linked to A, I would get something like this:
J, C, K, B, O, D, P, R, M
Since J is linked to A and B, K is linked to A, and C is linked to A and O and O is linked to D, and D is linked to O and P, R is linked to D and P is linked to B which is linked to M (and J).
Hmmm…my contrived example exploded on me more than I intended…
Mike Dewhirst
>
> I have a Document model that has an M2M-symmetrical field on itself.
> The idea is that we can “link” documents to other documents. It’s
> very simple to find the linked documents of a particular document, but
> is there an easy way to get all the linked documents of the linked
> documents of the document in question? I will probably have to write
> my own QuerySet or ModelManager function, but I was just curious if
> Django had something built in.
>
> For example:
>
> A is linked to J, C, K.
>
> èJ is linked to A
>
> èC is linked to A
>
> B is linked to J, M, P.
>
> èJ is linked to B
>
> èM is linked to B
>
> C is linked to O.
>
> D is linked to O, P, R.
>
> èO is linked to D
>
> èP is linked to D
>
> So when I get the entire “chain” of documents linked to A, I would get
> something like this:
>
> J, C, K, B, O, D, P, R, M
>
respect to itself. To do this I would nominate the "through" table and
have a model class which maps to it. That model will have two PKs to the
document model. The document model method(s) which return querysets
query the "through" table.
Also have a look at select_related
https://docs.djangoproject.com/en/1.10/ref/models/querysets/#select-related
> Since J is linked to A and B, K is linked to A, and C is linked to A
> and O and O is linked to D, and D is linked to O and P, R is linked to
> D and P is linked to B which is linked to M (and J).
>
> Hmmm…my contrived example exploded on me more than I intended…
>
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