postgresql - SQLAlchemy support of Postgres Schemas

Question

We host a multitenant app with SQLAlchemy and postgres. I am looking at moving from having separate databases for each tenant to a single database with multiple schemas. Does SQLAlchemy support this natively? I basically just want every query that comes out to be prefixed with a predetermined schema... e.g

select * from client1.users

instead of just

select * from users

Note that I want to switch the schema for all tables in a particular request/set of requests, not just a single table here and there.

I imagine that this could be accomplished with a custom query class as well but I can't imagine that something hasn't been done in this vein already.

Answer 1

well there's a few ways to go at this and it depends on how your app is structured. Here is the most basic way:

meta = MetaData(schema="client1")

If the way your app runs is one "client" at a time within the whole application, you're done.

But what may be wrong with that here is, every Table from that MetaData is on that schema. If you want one application to support multiple clients simultaneously (usually what "multitenant" means), this would be unwieldy since you'd need to create a copy of the MetaData and dupe out all the mappings for each client. This approach can be done, if you really want to, the way it works is you'd access each client with a particular mapped class like:

client1_foo = Client1Foo()

and in that case you'd be working with the "entity name" recipe at http://www.sqlalchemy.org/trac/wiki/UsageRecipes/EntityName in conjunction with sometable.tometadata() (see http://docs.sqlalchemy.org/en/latest/core/metadata.html#sqlalchemy.schema.Table.tometadata).

So let's say the way it really works is multiple clients within the app, but only one at a time per thread. Well actually, the easiest way to do that in Postgresql would be to set the search path when you start working with a connection:

# start request

# new session
sess = Session()

# set the search path
sess.execute("SET search_path TO client1")

# do stuff with session

# close it.  if you're using connection pooling, the
# search path is still set up there, so you might want to 
# revert it first
sess.close()

The final approach would be to override the compiler using the @compiles extension to stick the "schema" name in within statements. This is doable, but would be tricky as there's not a consistent hook for everywhere "Table" is generated. Your best bet is probably setting the search path on each request.