Mongoose/Mongo find in array of objectIds

I have this item in mongo:

[ 
  { 
    title: 'Product Name',
    _id: 5052843e023273693300013c,
    description: 'This is a fake description',
    categories: [ 5052843e023273693300010a ],
  }
]

I want to find products like this that have this category. I have tried:

Product.find({ categories:  mongoose.Types.ObjectId('5052843e023273693300010a')})
Product.find({ categories:  mongoose.mongo.BSONPure.ObjectID.fromString('5052843e023273693300010a')})
Product.find({ categories:  '5052843e023273693300010a'})
Product.find({ 'categories':  '5052843e023273693300010a'})
Product.find({ categories:  {$in: ['5052843e023273693300010a']}})
Product.find({ categories:  Schema.Types.ObjectId('5052843e023273693300010a')})

But nothing works. I can fetch by id just fine using: _id: '5052843e023273693300013c'.

Note that when the products were inserted the category ID was added as a string (meaning I just assigned the ID instead of the category objects but that doesn't explain why none of the above work - it's unquoted in the dump so perhaps Mongo recognizes as an object ID.

Similar questions on SO did not yield an answer.

I am using the latest Mongoose (3 something) and recent Mongo, Node.

Update:

I can fetch just fine from CLI using:

db.products.find({ categories: '5052843e02327369330000fe' }); 

and interestingly I can fetch it by doing the not equal in my code - huh?:

Product.find({ categories: { $ne: '5052843e02327369330000fe' }})

My schema is as follows:

var Product = new Schema({
    title: { type: String, required: true },
    slug: { type: String },
    summary: { type: String }, //browser title
    description: { type: String, required: false },
    excerpt: { type: String },    //for a list and also for the meta description
    publish: { type: Boolean },
    featured: { type: Boolean },
    unavail