Combining results of two tables in mongoid/mongo - ruby-on-rails

Question

Hi guys what would be the best way to combine results of two mongoid queries.
My issue is that I would like to know active users, A user can send a letter and a notification, both are separate table and a user if he sends either the letter or the notification is considered active. What I want to know is how many active users were there per month.
right now what I can think of is doing this
Letter.collection.aggregate([
{ '$match': {}.merge(opts) },
{ '$sort': { 'created_at': 1 } },
{
'$group': {
_id: '$customer_id',
first_notif_sent: {
'$first': {
'day': { '$dayOfMonth': '$created_at' },
'month': { '$month': '$created_at' },
'year': { '$year': '$created_at' }
}
}
}
}])
Notification.collection.aggregate([
{ '$match': {}.merge(opts) },
{ '$sort': { 'created_at': 1 } },
{
'$group': {
_id: '$customer_id',
first_notif_sent: {
'$first': {
'day': { '$dayOfMonth': '$created_at' },
'month': { '$month': '$created_at' },
'year': { '$year': '$created_at' }
}
}
}
}])
What I am looking for is to get the minimum of the dates and then combine the results and get the count. Right now I can get the results and loop over each of them and create a new list. But I wanted to know if there is a way to do it in mongo directly.
EDIT
For letters
def self.get_active(tenant_id)
map = %{
function() {
emit(this.customer_id, new Date(this.created_at))
}
}
reduce = %{
function(key, values) {
return new Date(Math.min.apply(null, values))
}
}
where(tenant_id: tenant_id).map_reduce(map, reduce).out(reduce: "#{tenant_id}_letter_notification")
end
Notifications
def self.get_active(tenant_id)
map = %{
function() {
emit(this.customer_id, new Date(this.updated_at))
}
}
reduce = %{
function(key, values) {
return new Date(Math.min.apply(null, values))
}
}
where(tenant_id: tenant_id, transferred: true).map_reduce(map, reduce).out(reduce: "#{tenant_id}_outgoing_letter_standing_order_balance")
end
This is what I am thinking of going with, one of the reason is that, lookup does not work with my version of mongo.

Answer 1

the customer created a new notification, or a new letter, and I would like to get the first created at of either.
Let's address this first as a foundation. Given examples of document schema as below:
Document schema in Letter collection:
{ _id: <ObjectId>,
customer_id: <integer>,
created_at: <date> }
And, document schema in Notification collection:
{ _id: <ObjectId>,
customer_id: <integer>,
created_at: <date> }
You can utilise aggregation pipeline $lookup to join the two collections. For example using mongo shell :
db.letter.aggregate([
{"$group":{"_id":"$customer_id", tmp1:{"$max":"$created_at"}}},
{"$lookup":{from:"notification",
localField:"_id",
foreignField:"customer_id",
as:"notifications"}},
{"$project":{customer_id:"$_id",
_id:0,
latest_letter:"$tmp1",
latest_notification: {"$max":"$notifications.created_at"}}},
{"$addFields":{"latest":
{"$cond":[{"$gt":["$latest_letter", "$latest_notification"]},
"$latest_letter",
"$latest_notification"]}}},
{"$sort":{latest:-1}}
], {cursor:{batchSize:100}})
The output of the above aggregation pipeline is a list of customers in sorted order of created_at field from either Letter or Notification. Example output documents:
{
"customer_id": 0,
"latest_letter": ISODate("2017-12-19T07:00:08.818Z"),
"latest_notification": ISODate("2018-01-26T13:43:56.353Z"),
"latest": ISODate("2018-01-26T13:43:56.353Z")
},
{
"customer_id": 4,
"latest_letter": ISODate("2018-01-04T18:55:26.264Z"),
"latest_notification": ISODate("2018-01-25T02:05:19.035Z"),
"latest": ISODate("2018-01-25T02:05:19.035Z")
}, ...
What I want to know is how many active users were there per month
To achieve this, you can just replace the last stage ($sort) of the above aggregation pipeline with $group. For example:
db.letter.aggregate([
{"$group":{"_id":"$customer_id", tmp1:{$max:"$created_at"}}},
{"$lookup":{from:"notification",
localField:"_id",
foreignField:"customer_id",
as:"notifications"}},
{"$project":{customer_id:"$_id",
_id:0,
latest_letter:"$tmp1",
latest_notification: {"$max":"$notifications.created_at"}}},
{"$addFields":{"latest":
{"$cond":[{"$gt":["$latest_letter", "$latest_notification"]},
"$latest_letter",
"$latest_notification"]}}},
{"$group":{_id:{month:{"$month": "$latest"},
year:{"$year": "$latest"}},
active_users: {"$sum": "$customer_id"}
}
}
],{cursor:{batchSize:10}})
Where the example output would be as below:
{
"_id": {
"month": 10,
"year": 2017
},
"active_users": 9
},
{
"_id": {
"month": 1,
"year": 2018
},
"active_users": 18
},