[{"_id":1,"name":"Sarah","fruitEaten":["apple","banana","orange"]},{"_id":2,"name":"Steve","fruitEaten":["apple"]},{"_id":3,"name":"Stacey","fruitEaten":["apple","orange"]},{"_id":4,"name":"NotArray","fruitEaten":"apple"}]

db.collection.aggregate([{$match:{$expr:{$and:[{$isArray:"$fruitEaten"},{$gt:[{$size:"$fruitEaten"},1]}]}}}])