Develop Reference

Extracting and updating a dictionary from array of dictinaries in MongoDB

I have a structure like this:
{
"id" : 1,
"user" : "somebody",
"players" : [
{
"name" : "lala",
"surname" : "baba",
"player_place" : "1",
"start_num" : "123",
"results" : {
"1" : { ... }
"2" : { ... },
...
}
},
...
]
}
I am pretty new to MongoDB and I just cannot figure out how to extract results for a specific user (in this case "somebody", but there are many other users and each has an array of players and each player has many results) for a specific player with start_num.
I am using pymongo and this is the code I came up with:
record = collection.find(
{'user' : name}, {'players' : {'$elemMatch' : {'start_num' : start_num}}, '_id' : False}
)
This extracts players with specific player for a given user. That is good, but now I need to get specific result from results, something like this:
{ 'results' : { '2' : { ... } } }.
I tried:
record = collection.find(
{'user' : name}, {'players' : {'$elemMatch' : {'start_num' : start_num}}, 'results' : result_num, '_id' : False}
)
but that, of course, doesn't work. I could just turn that to list in Python and extract what I need, but I would like to do that with query in Mongo.
Also, what would I need to do to replace specific result in results for specific player for specific user? Let's say I have a new result with key 2 and I want to replace existing result that has key 2. Can I do it with same query as for find() (just replacing method find with method replace or find_and_replace)?

You can replace a specific result and the syntax for that should be something like this,
assuming you want to replace the result with key 1,
collection.updateOne({
"user": name,
"players.start_num": start_num
},
{ $set: { "players.$.results.1" : new_result }})

Delete a list item from an Embedded Document MongoEngine

I'm having trouble to find the way to delete a list item inside an Embedded document using mongo engine.
Document structure:
{
"_id" : "id",
"user" : ObjectId("xxxxxxx"),
"invoice" : false,
"coupon" : ObjectId("xxxxxxx"),
"date" : ISODate("2017-03-31T11:32:57.467Z"),
"orders" : [
{
"products" : [
ObjectId("xxxxxxx")
]
}
],
"shipping_address" : {
},
}
Using this structure, what I want to achieve, is to delete the id inside the products list inside the orders embedded document.
Any idea? any help would be really appreciated

update nth document in a nested array document in mongo

I need to update a document in an array inside another document in Mongo DB.
{
"_id" : ObjectId("51cff693d342704b5047e6d8"),
"author" : "test",
"body" : "sdfkj dsfhk asdfjad ",
"comments" : [
{
"author" : "test",
"body" : "sdfkjdj\r\nasdjgkfdfj",
"email" : "test#tes.com"
},
{
"author" : "hola",
"body" : "sdfl\r\nhola \r\nwork here"
}
],
"date" : ISODate("2013-06-30T09:12:51.629Z"),
"permalink" : "mxwnnnqafl",
"tags" : [
"ab"
],
"title" : "cd"
}
If I try to update first document in comments array by below command, it works.
db.posts.update({'permalink':"cxzdzjkztkqraoqlgcru"},{'$inc': {"comments.0.num_likes": 1}})
But if I put the same in python code like below, I am getting Write error, that it can't traverse the element. I am not understanding what is missing!!
Can anyone help me out please.
post = self.posts.find_one({'permalink': permalink})
response = self.posts.update({'permalink': permalink},
{'$inc':"comments.comment_ordinal.num_likes": 1}})
WriteError: cannot use the part (comments of comments.comment_ordinal.num_likes) to traverse the element

comment_ordinal should be a substitution, not the index itself. You're treating it like an integer that can be mapped to an ordinal number. I mean you should do something like:
updated_field = "comments." + str(comment_ordinal) + ".num_likes"
response = self.posts.update({'permalink': permalink}, {'$inc': {updated_field: 1}})
Hope this helps.

You are doing it wrong you need to build your query dynamically and the best way to do that is using the str.format method.
response = self.posts.update_one(
{'permalink': permalink},
{'$inc': {"comments.{}.num_likes".format(comment_ordinal): 1}}
)
Also you should consider to use the update_one method for single update and update_many if you need to update multiple documents because update is deprecated.

Pull from a list in a dict using mongoengine

I have this Document in mongo engine:
class Mydoc(db.Document):
x = db.DictField()
item_number = IntField()
And I have this data into the Document
{
"_id" : ObjectId("55e360cce725070909af4953"),
"x" : {
"mongo" : [
{
"list" : "lista"
},
{
"list" : "listb"
}
],
"hello" : "world"
},
"item_number" : 1
}
Ok if I want to push to mongo list using mongoengine, i do this:
Mydoc.objects(item_number=1).update_one(push__x__mongo={"list" : "listc"})
That works pretty well, if a query the database again i get this
{
"_id" : ObjectId("55e360cce725070909af4953"),
"x" : {
"mongo" : [
{
"list" : "lista"
},
{
"list" : "listb"
},
{
"list" : "listc"
}
],
"hello" : "world"
},
"item_number" : 1
}
But When I try to pull from same list using pull in mongo engine:
Mydoc.objects(item_number=1).update_one(pull__x__mongo={'list': 'lista'})
I get this error:
mongoengine.errors.OperationError: Update failed (Cannot apply $pull
to a non-array value)
comparising the sentences:
Mydoc.objects(item_number=1).update_one(push__x__mongo={"list" : "listc"}) # Works
Mydoc.objects(item_number=1).update_one(pull__x__mongo={"list" : "listc"}) # Error
How can I pull from this list?
I appreciate any help

I believe that the problem is that mongoengine doesn't know the structure of your x document. You declared it as DictField, so mongoengine thinks you are pulling from DictField not from ListField. Declare x as ListField and both queries should work just fine.
I suggest you should also create an issue for this:
https://github.com/MongoEngine/mongoengine/issues
As a workaround, you can use a raw query:
Mydoc.objects(item_number=1).update_one(__raw__={'$pull': {'x.mongo': {'list': 'listc'}}})

$addToSet nested nested object using pymongo

As part of a for-in loop in python using pymongo I want to add some nested documents/objects inside a linktype field which is to be within a links field:
neither the links field or linktype field exists before the first such entries are to be added.
What is the commands to do this ?
Here is an item before adding links:
item = {
"_id" : ObjectId("5067c26b9d595266e25e825a"),
"name": "a Name"
}
And after adding one link of type typeA:
toType = "typeA"
to_link = {"_id" : ObjectId("5067c26b9d595266e25e825b"), "property":"value"}
{
"_id" : ObjectId("5067c26b9d595266e25e825a"),
"name": "a Name",
"links" : {
"typeA":{
{"_id" : ObjectId("5067c26b9d595266e25e825b"), "property":"value"}
}
}
}
I have tried:
db.collection.update({"name":"a Name"},{{"links":{"$addToSet":{toType:to_link}}})
which doesnt work.
If I just use:
db.collection.update({"name":"a Name"},{ {"$addToSet":{toType:to_link}} )
that works but that is not what i want.

$addToSet is for adding to an array. To add a new property to an existing embedded object you need to use the $set operator and dot notation as:
db.collection.update({name: 'a name'}, {$set: {'links.' + toType: to_link}})