If the server receives a document for insert that is missing the _id field, it should generate one. A bug in 2.6 caused mongos not to create these _id values. This can violate the assumption that all documents have an _id and can lead to errors when inserting into a collection sharded on _id.
Minor, as all drivers automatically generate the _id. Some drivers (for example the Python driver) have an option to disable generating the _id field client-side. Using this option can cause issues when inserting documents to a sharded collection on _id through a mongos.
Use the driver default to auto-generate _id values.
This patch will cause mongos to auto-generate _id values if they are not present.
Version 2.6.0 is affected by this bug.
The patch is included in the 2.6.1 production release.