PyMongo - db.coll.CopyTo

[marc.m...@upf.edu]

Hi,

I would like to know if there is an equivalent or workaround for the command

db.collection.CopyTo("newcollection")

in PyMongo.

Thanks

[Bernie Hackett]

No, but you could easily write the same thing using PyMongo. You can see how any function in the mongo shell is implemented through it name. From the mongo shell:

> db.foo.copyTo

function ( newName ){

    return this.getDB().eval(

        function( collName , newName ){

            var from = db[collName];

            var to = db[newName];

            to.ensureIndex( { _id : 1 } );

            var count = 0;

            var cursor = from.find();

            while ( cursor.hasNext() ){

                var o = cursor.next();

                count++;

                to.save( o );

            }

            return count;

        } , this.getName() , newName

    );

}

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[Paulie Pena]

Since MongoDB just uses eval() within the copyTo() call, you could also just use eval() in pymongo, e.g.

db.eval('db.collection.copyTo("newcollection")')

[Stephen Steneker]

Hi Paulie,

Your suggestion won't work.. a client-side eval() in the mongo shell is very different from running the server side eval() via Python. For example, there is no access to a "db" object within a server-side eval().

To mimic the mongo shell's db.collection.copyTo() via PyMongo, you will have to implement the same client-side logic as suggested by Bernie.

Regards,

Stephen

[Paulie Pena]

I've tried it using MongoDB version 2.2.2 and pymongo 2.6.3, and it works fine for me.

-Paulie

[Paulie Pena]

I just realized that db.eval() creates a global lock which prevents reads and writes to the server, so copyTo() is probably not the safest thing to use, both because Python would need to use db.eval() to call it but also because copyTo() itself calls db.eval() (unless you really want to stop all reads and writes while the collection is copying). Instead of copyTo() and db.eval(), you can prevent the global read/write lock by using db.runCommand({eval:..., nolock:true}), which in Python would be like this (see http://api.mongodb.org/python/2.2/api/pymongo/database.html#pymongo.database.Database.command):

db.command('eval', str_javascript_func, nolock=True)