MongoDB aggregate. Create new groups for non-existing items - mongodb

Question

My collection of documents contains information about users, their sessions and CRUD operations they performed during these sessions:
{
user_id: '1',
sessions: [
{
actions: [
{
type: 'create',
created_at: ISODate('2020-01-01T00:00:00'),
},
{
type: 'read',
created_at: ISODate('2022-01-01T00:00:00'),
},
{
type: 'read',
created_at: ISODate('2021-01-01T00:00:00'),
}
],
}
]
}
I need to get a summary for each user, which includes the amount of CRUD operations and the date of the last one:
{
user_id: '1',
actions: [
{
type: 'create',
last: ISODate('2020-01-01T00:00:00'),
count: 1,
},
{
type: 'read',
last: ISODate('2022-01-01T00:00:00'),
count: 2,
},
// Problematic part:
{
type: 'update',
last: null,
count: 0,
},
{
type: 'delete',
last: null,
count: 0,
},
]
}
I came up with this solution:
db.users.aggregate([
{$unwind:'$sessions'},
{$unwind:'$sessions.actions'},
{
$group:{
_id:{user_id:'$user_id', type:'$sessions.actions.type'},
last:{$max:'$sessions.actions.created_at'},
count:{$sum:1},
}
},
{
$group:{
_id:{user_id:'$_id.user_id'},
actions:{$push:{type:'$_id.type', last:'$last', count:'$count'}}
}
},
{
$project:{
_id:0,
user_id: '$_id.user_id',
actions: '$actions'
}
}
])
The problem here is that I cannot figure out, how can I add missing actions, like in 'update' and 'delete' in the example above

Answer 1

Try this,
db.collection.aggregate([
{
$unwind: "$sessions"
},
{
$unwind: "$sessions.actions"
},
{
$group: {
_id: {
user_id: "$user_id",
type: "$sessions.actions.type"
},
last: {
$max: "$sessions.actions.created_at"
},
count: {
$sum: 1
},
}
},
{
$group: {
_id: {
user_id: "$_id.user_id"
},
actions: {
$push: {
type: "$_id.type",
last: "$last",
count: "$count"
}
}
}
},
{
$project: {
_id: 0,
user_id: "$_id.user_id",
actions: {
"$function": {
"body": "function(doc) { const ops = {read:0, delete:0, update: 0, create: 0}; const actions = doc.actions; actions.forEach(action => { ops[action.type] = 1 }); Object.keys(ops).filter(key => ops[key] === 0).forEach(key => actions.push({count: 0, last: null, type: key})); return actions }",
"args": [
"$$ROOT"
],
"lang": "js"
}
},
}
},
])
Here, we use $function and provide a small JS function to populate the missing entries.
Playground link.