>From the Django docs, for any ORM query made, the DB alias to use is
determined by the following rules:
- Checks if the using keyword is used either as a parameter in the
function call or chained with QuerySet.
- Consults the DB routers in order until a match is found.
- Falls back to the default router which always returns default as the
alias.
Using the router scheme works perfectly for ORM queries. However, when it
comes to using transaction APIs (like the transaction.atomic construct),
it becomes mandatory to specify the *using* kwarg explicitly in all of its
publicly exposed APIs if a DB other than the default alias has to be chosen.
To illustrate why this is a problem, consider the following scenario:
- A DB router exists such that it directs queries to a specific database
based on a value set in *thread-local* storage by a middleware.
- When ORM queries from within the view are fired, they automatically
get forwarded to the right DB *without explicitly citing* the using keyword
owing to the router.
- But if the transaction.atomic construct is used, the using keyword
would have to be explicitly specified each time. While this might seem
fine, it creates the following problems:
1. For large code bases, it becomes highly unwieldy to make sure that
the *using* keyword has been mentioned in every transaction API call.
Also, if one tries to implement the above scheme in an already existing
code base, it would be impractical to add the using keyword in all lines
calling the transaction APIs.
2. It doesn't gel well with the the routers framework.
A proposed workaround could to be to add a separate method named
*db_for_transaction* to the routers framework which would be consulted by
the transaction APIs first, before falling back to using the default DB
alias.
1. Having a separate method for transaction is good because it need not
be mangled up with the other methods i.e db_for_read / db_for_write as they
clearly indicate certain operational semantics which aren't tied to a
transaction since it can have both reads and writes within it.
2. Also, if in the future, there is some special info that can be
delivered to the transaction based on which a decision regd which DB to use
could be made, then it would be cleanly isolated into its own method.
If we can finalise on this, I could submit a PR for the same.
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