Our tests in the jstests/core/ directory tend to take one of the following two forms.

// DB.prototype.getCollection() is called before DBCollection.prototype.drop(), and the local
// variable is used when inserting a document into the collection.
var coll = db.mycoll_var;
coll.drop();
assert.writeOK(coll.insert({}));

// DB.prototype.getCollection() is called before DBCollection.prototype.drop(), and the cached
// DBCollection object is used when inserting a document into the collection.
db.mycoll_novar.drop();
assert.writeOK(db.mycoll_novar.insert({}));

Tests drop the collection that they act as one of their first operations so that they may be re-run without having to clear the data directory. This poses an issue for the current implementation of the sharded_collections_jscore_passthrough suite because the test will proceed with the collection not being sharded after the collection has been dropped. We should consider overriding DBCollection.prototype.drop() (or the Mongo.prototype.runCommand() equivalent) in implicitly_shard_accessed_collections.js to gain coverage in running a variety of operations against a sharded collection.

Note: the value previously returned by the DB.prototype.getCollection() is cached, the DB.prototype.getCollection() function will not be called a second time.