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This one has been bugging us for a while at work, so it seemed like a good one to blog about.

TypeError: Cannot read property 'raw' of undefined
    at [object Object].<anonymous> ([...]node_modules/mongoose/node_modules/mongodb/lib/mongodb/connection/server.js:436:77)
    at EventEmitter.emit (events.js:95:17)
    at [object Object].<anonymous> ([...]node_modules/mongoose/node_modules/mongodb/lib/mongodb/connection/connection_pool.js:201:13)
    at EventEmitter.emit (events.js:98:17)
    at Socket.<anonymous> ([...]node_modules/mongoose/node_modules/mongodb/lib/mongodb/connection/connection.js:439:22)
    at EventEmitter.emit (events.js:95:17)[...]

Debugging this involved a bit of luck, particularly in finding a way to reproduce it outside our production environment, and then again in getting it to go away. That the error occurred in a sub-package outside the context of the main application made it difficult to tie back to a particular piece of our code.

The problem stemmed from the use of the MongoDB database driver in the main app, separately from Mongoose. Earlier work we did sought to establish a single database connection that could be shared between code using Mongoose and code using MongoDB directly. The way we did this was by establishing a connection through Mongoose, then passing the database client object out for use by the MongoDB driver.

  var mongooseDB = mongoose.connect(process.env.MONGO_URL, { db: { w: 'majority' }})
  var db = _.first(mongooseDB.connections).db

Elsewhere, we created Collections objects using db, with calls to the local MongoDB package, rather than the one inside Mongoose. That resulted in a consistency problem between the package used by Mongoose and the local package. When the database sent a response to a query, the handler for that connection couldn't be found by the driver that responded to it, hence the error.

The solution was to change require("mongodb") to require("mongoose/node_modules/mongodb") everywhere that we included it. That gave us a single, shared instance of the MongoDB driver and the errors stopped.