node.js - Mongoose - get length of array in model -

i have mongoose schema:

var postschema = mongoose.schema({      postid: {         type: number,         unique: true     },      upvotes: [         {             type: number,             unique: true         }     ]  }); 

what best query use length of upvotes array? don't believe need use aggregation because want query 1 model, need length of upvotes array given model.

really struggling find info online - search mentions aggregation methodology don't believe need.

also, side note, unique schema property of upvotes array doesn't work, perhaps doing wrong.

find results can include content docs themselves¹, while aggregate can project new values derived doc's content (like array's length). that's why need use aggregate this, though you're getting single doc.

post.aggregate([{$match: {postid: 5}}, {$project: {upvotes: {$size: '$upvotes'}}}]) 

¹_{single exception $meta projection operator project $text query result's score.}