Re: [sqlalchemy] Using the declarative base across projects
Hi,
Just wanted to say thanks to those who helped me with this. Simon's solution
was exactly what I was looking for (though I have to admit I don't exactly
understand *how* it works!). But that's no longer an SQLAlchemy question...
Cheers,
Demitri
On Jul 8, 2010, at 5:49 AM, King Simon-NFHD78 wrote:
In general, you don't need a database connection just to define your
tables and mappers. The 'bind' parameter to DeclarativeBase is optional,
and only necessary if you are using autoloading. So one solution to your
problem would be not to use autoloading, and bind to a database at the
Session level rather than the Mapper level. That would be the usual way
to use the same set of classes against multiple databases.
If you really need to use autoloading, you could move all your class
definitions into a function that accepts a database engine as a
parameter. For example:
#
# ModelClasses.py
class Namespace(object):
def __init__(self, **kwargs):
self.__dict__.update(kwargs)
def initdb(connection_string):
engine = create_engine(connection_string)
Base = declarative_base(bind=engine)
class Table1(Base):
__tablename__ = 'table1'
__table_args__ = {'autoload': True}
return Namespace(Base=Base,
Table1=Table1)
# or, you could be lazy:
# return Namespace(**locals())
# MainScript1.py
import ModelClasses
db = ModelClasses.initdb(my_connection_string)
# access db.Table1, db.Base etc.
Hope that helps,
Simon
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Re: [sqlalchemy] Using the declarative base across projects
thatsanicehatyouh...@mac.com wrote: This is a bit tricky to explain. Imagine I have one database, and I create a project (1) to work with that database (connections, table class definitions, etc.). That is standalone (to me). I have another completely separate database (2) on another host where I do the same thing. Using replication I then create a read-only copy of database 1 in database 2, and join some of the tables. Project 2 needs to generate the classes, but use SA's Base class that is dynamically generated. Since it's dynamic, I have to create it at run time... but now I can't pass that to the definition of project 1's classes. It's that communication that I'm struggling with. Difficult to know what you're after so two guesses: - if you have multiple python packages that need to share a declarative Base, either have one master package that defines the Base and import from that or, if you prefer completely normalised dependencies, move the Base definition out its own package (where stuff shared between your packages such as mixins, session setup functions, etc can live) and have both packages import from there. - if you have different python packages on two different projects running on two different machines or in different processes, then just have each create their own Base. The Base is merely a collection of references to the SA mapped classes, much like the MetaData object, so it's fine to have different ones in different processes, even if they're both accessing the same tables in the same databases... cheers, Chris -- Simplistix - Content Management, Batch Processing Python Consulting - http://www.simplistix.co.uk -- You received this message because you are subscribed to the Google Groups sqlalchemy group. To post to this group, send email to sqlalch...@googlegroups.com. To unsubscribe from this group, send email to sqlalchemy+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/sqlalchemy?hl=en.
RE: [sqlalchemy] Using the declarative base across projects
-Original Message-
From: sqlalchemy@googlegroups.com
[mailto:sqlalch...@googlegroups.com] On Behalf Of
thatsanicehatyouh...@mac.com
Sent: 07 July 2010 20:33
To: sqlalchemy@googlegroups.com
Subject: Re: [sqlalchemy] Using the declarative base across projects
Hi Lance,
Thanks for your comments.
On Jul 7, 2010, at 12:28 PM, Lance Edgar wrote:
Why not just do this in project2 ?
import project.DatabaseConnection as db
Base = declarative_base(bind=db.engine)
# ... etc.
The DatabaseConnection class contains the particulars of the
connection (i.e. host, username, password) which can be
different. I don't want to create dependencies between the
projects, I just want to reuse the class definitions. I want
to create the DatabaseConnection once and pass it into the
definition of the classes.
Another approach I tried was to make ModelClasses an object
and define the classes in a method there (so I could just
pass the Base class to it), but the class definitions were in
the wrong namespace.
The python way seems to be to create a config class,
but project2.ModelClasses won't know anything about it if
it's defined in the first project. As to the reason why there
are two separate projects, consider the case where one set of
tables is one logical group, and the second is a replicated
copy from another server. I can't merge all of these projects
since they really are independent units, but sometimes I
will link them (as above).
I don't understand why project2 wouldn't know anything
about it if defined in (first) project. All it needs to do
is import the connection info from the project (as in above
example). If the database configuration really transcends
both project and project2 though, then yes it probably could
be wrapped in a config module of some sort in another
project; depending on the scope that may be a bit overkill.
If you can consider either project or project2 to be
slightly more default than the other then the db config could
stay there I'd think.
This is a bit tricky to explain. Imagine I have one database,
and I create a project (1) to work with that database
(connections, table class definitions, etc.). That is
standalone (to me). I have another completely separate
database (2) on another host where I do the same thing. Using
replication I then create a read-only copy of database 1 in
database 2, and join some of the tables. Project 2 needs to
generate the classes, but use SA's Base class that is
dynamically generated. Since it's dynamic, I have to create
it at run time... but now I can't pass that to the definition
of project 1's classes. It's that communication that I'm
struggling with.
Cheers,
Demitri
In general, you don't need a database connection just to define your
tables and mappers. The 'bind' parameter to DeclarativeBase is optional,
and only necessary if you are using autoloading. So one solution to your
problem would be not to use autoloading, and bind to a database at the
Session level rather than the Mapper level. That would be the usual way
to use the same set of classes against multiple databases.
If you really need to use autoloading, you could move all your class
definitions into a function that accepts a database engine as a
parameter. For example:
#
# ModelClasses.py
class Namespace(object):
def __init__(self, **kwargs):
self.__dict__.update(kwargs)
def initdb(connection_string):
engine = create_engine(connection_string)
Base = declarative_base(bind=engine)
class Table1(Base):
__tablename__ = 'table1'
__table_args__ = {'autoload': True}
return Namespace(Base=Base,
Table1=Table1)
# or, you could be lazy:
# return Namespace(**locals())
# MainScript1.py
import ModelClasses
db = ModelClasses.initdb(my_connection_string)
# access db.Table1, db.Base etc.
Hope that helps,
Simon
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Re: [sqlalchemy] Using the declarative base across projects
On Wed, 2010-07-07 at 11:13 -0400, thatsanicehatyouh...@mac.com wrote:
Hi,
I have a question that I can't find a satisfactory answer to. Apologies in
advance if it's more of a Python question, but it's possible that there is a
SA solution.
I have a project that defines a database connection and classes based on
database tables. A script that uses these classes would start with:
import project.DatabaseConnection as db # create_engine() and metadata
defined here
import project.ModelClasses
In ModelClasses, I define all of the table classes using:
Base = declarative_base(bind=db.engine)
class Table1(Base):
__tablename__ = 'table1'
__table_args__ = {'autoload' : True} # requires metadata to work
This is fine. I have a second project that also defines a different set of
tables that I want to use, so the main script would then:
import project2.ModelClasses # a different set of tables
If it's not clear how the db parameter (the database connection) was
defined in ModelClasses, well, that's my problem. I can't pass a parameter to
an import statement of course. The DatabaseConnection class defines the
engine and metadata, and now I need to use these objects to generate my base
class. How can I pass this object around? Should I be using a different model?
Why not just do this in project2 ?
import project.DatabaseConnection as db
Base = declarative_base(bind=db.engine)
# ... etc.
The python way seems to be to create a config class, but
project2.ModelClasses won't know anything about it if it's defined in the
first project. As to the reason why there are two separate projects, consider
the case where one set of tables is one logical group, and the second is a
replicated copy from another server. I can't merge all of these projects
since they really are independent units, but sometimes I will link them (as
above).
I don't understand why project2 wouldn't know anything about it if
defined in (first) project. All it needs to do is import the connection
info from the project (as in above example). If the database
configuration really transcends both project and project2 though, then
yes it probably could be wrapped in a config module of some sort in
another project; depending on the scope that may be a bit overkill. If
you can consider either project or project2 to be slightly more
default than the other then the db config could stay there I'd think.
Lance
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Re: [sqlalchemy] Using the declarative base across projects
Hi Lance, Thanks for your comments. On Jul 7, 2010, at 12:28 PM, Lance Edgar wrote: Why not just do this in project2 ? import project.DatabaseConnection as db Base = declarative_base(bind=db.engine) # ... etc. The DatabaseConnection class contains the particulars of the connection (i.e. host, username, password) which can be different. I don't want to create dependencies between the projects, I just want to reuse the class definitions. I want to create the DatabaseConnection once and pass it into the definition of the classes. Another approach I tried was to make ModelClasses an object and define the classes in a method there (so I could just pass the Base class to it), but the class definitions were in the wrong namespace. The python way seems to be to create a config class, but project2.ModelClasses won't know anything about it if it's defined in the first project. As to the reason why there are two separate projects, consider the case where one set of tables is one logical group, and the second is a replicated copy from another server. I can't merge all of these projects since they really are independent units, but sometimes I will link them (as above). I don't understand why project2 wouldn't know anything about it if defined in (first) project. All it needs to do is import the connection info from the project (as in above example). If the database configuration really transcends both project and project2 though, then yes it probably could be wrapped in a config module of some sort in another project; depending on the scope that may be a bit overkill. If you can consider either project or project2 to be slightly more default than the other then the db config could stay there I'd think. This is a bit tricky to explain. Imagine I have one database, and I create a project (1) to work with that database (connections, table class definitions, etc.). That is standalone (to me). I have another completely separate database (2) on another host where I do the same thing. Using replication I then create a read-only copy of database 1 in database 2, and join some of the tables. Project 2 needs to generate the classes, but use SA's Base class that is dynamically generated. Since it's dynamic, I have to create it at run time... but now I can't pass that to the definition of project 1's classes. It's that communication that I'm struggling with. Cheers, Demitri -- You received this message because you are subscribed to the Google Groups sqlalchemy group. To post to this group, send email to sqlalch...@googlegroups.com. To unsubscribe from this group, send email to sqlalchemy+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/sqlalchemy?hl=en.
