Mongo: how to sort by external weight

Question

Following this question which @NeilLunn has gracefully answered, here is my problem in more detail.

This is the set of documents, some have user_id some don\'t. The

Answer 1

Given the array [4,7,90,1] what you want in your query is this:

db.collection.aggregate([
   { "$project": {
       "user_id": 1,
       "content": 1,
       "date": 1,
       "weight": { "$or": [
           { "$eq": ["$user_id": 4] }, 
           { "$eq": ["$user_id": 7] }, 
           { "$eq": ["$user_id": 90] }, 
           { "$eq": ["$user_id": 1] }, 
       ]}
   }},
   { "$sort": { "weight": -1, "date": -1 } }
])

So what that does is, for every item contained in that $or condition, the user_id field is tested against the supplied value, and $eq returns 1 or 0 for true or false.

What you do in your code is for each item you have in the array you build the array condition of $or. So it's just creating a hash structure for each equals condition, passing it to an array and plugging that in as the array value for the $or condition.

I probably should have left the $cond operator out of the previous code so this part would have been clearer.

Here's some code for the Ruby Brain:

userList = [4, 7, 90, 1];

orCond = [];

userList.each do |userId|
  orCond.push({ '$eq' => [ 'user_id', userId ] })
end

pipeline = [
    { '$project' => {
        'user_id' => 1,
        'content' => 1,
        'date' => 1,
        'weight' => { '$or' => orCond }
    }},
    { '$sort' => { 'weight' => -1, 'date' => -1 } }
]

If you want to have individual weights and we'll assume key value pairs, then you need to nest with $cond :

db.collection.aggregate([
   { "$project": {
       "user_id": 1,
       "content": 1,
       "date": 1,
       "weight": { "$cond": [
           { "$eq": ["$user_id": 4] },
           10,
           { "$cond": [ 
               { "$eq": ["$user_id": 7] },
               9,
               { "$cond": [
                   { "$eq": ["$user_id": 90] },
                   7,
                   { "$cond": [
                       { "$eq": ["$user_id": 1] },
                       8, 
                       0
                   ]}
               ]}
           ]}
       ]}
   }},
   { "$sort": { "weight": -1, "date": -1 } }
])

Note that it's just a return value, these do not need to be in order. And you can think about the generation of that.

For generating this structure see here:

https://stackoverflow.com/a/22213246/2313887