MongoDB aggregate to extract values from a nested array and return a total

Question

I have this collection structure:

[
   "_id": "61a013b59f9dd0ebfd23ftgb",
   "modules": [
     {
       "_id": "61a013b59f9dd0ebfd964dgh",
       "videos": [
         {
           "_id": "213412",
           "progress": 100
         },
         {
           "_id": "61a013b59f9dd0ebfd965f4a",
           "progress": 0
         },
       ]
     },
     {
       "_id": "43556hujferwdhgsdft",
       "videos": [
         {
           "_id": "fdsg3sg98er989890",
           "progress": 66
         },
         {
           "_id": "fdsg3sg98er989890",
           "progress": 100
         },
         {
           "_id": "fdsg3sg98er989890",
           "progress": 100
         }
       ]
     }
   ]
 ]

I am trying to return the overall progress for each "module" by adding up all the videos that have progress of 100 and creating a percentage based on number of videos in the module. For example, the first module should return "module_progess" of 50 within it as this has 1/2 videos completed.

{
   "_id": "61a013b59f9dd0ebfd964dgh",
   "module_progress": 50,
   "videos": [
     {
       "_id": "213412",
       "progress": 100
     },
     {
       "_id": "61a013b59f9dd0ebfd965f4a",
       "progress": 0
     },
   ]
},

How do i access each videos object to make this calculation and add the new field to the response?

Answer 1

Query1

• map on modules
• and add a field on each with the avg progress of the videos

Test code here

aggregate(
[{"$set": 
   {"modules": 
     {"$map":  
       {"input": "$modules",
        "in": 
        {"$mergeObjects": 
          ["$$this",
           {"module_progress": {"$avg": "$$this.videos.progress"}}]}}}}}])

Query2

• unwind
• replace root to make structure better
• add the avg field

Test code here

aggregate(
[{"$unwind": {"path": "$modules"}},
  {"$replaceRoot": {"newRoot": "$modules"}},
  {"$set": 
    {"module_progress": 
      {"$avg": 
        {"$map": 
          {"input": "$videos.progress",
            "in": {"$cond": [{"$eq": ["$$this", 100]}, "$$this", 0]}}}}}}])