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

Mongodb python syntax to get array value .

so please be gentle
Have a mongo doc like this :
{ "Institute" : "Ucambridge",
"Project" : [ #array of projects
{"Sample":[ #array of samples
{ "workflow" : "abc", "owner" : "peter" }
]
"pname":"project1",
"dir" : "C drive"
}
]
}
I am aware that having nested loops in mongo isn't a great idea , however this is the way the data is being handed to me.
Trying to loop over all my projects and extract the project name, on my python server.
so get cursor :
u = mongo.db.testpymongo.find( )
Can get Institute by :
for x in u :
print x["Institute"]
Can get project by :
for x in u :
print x["Project"]
which returns :
[{u'Sample':[{u'workflow:':u'wf', u'owner':u'peter'} ] u'pname':u'project1 ', u'dir:u'C drive'}]
but , how do i get access to just my pname variable from the cursor ?
i have tried :
1.print x["Project:pname"] # does not work
2.print x["Project":"pname"] # gives unhashable type error
3.print x["pname"] # gives Key error
4.print x["Project"].["pname"] # gives syntax error
5.print x["Project.pname"] # gives key error
Should i be using attributes in the find() function to only return part of the document ?
i.e : like so ?
d = mongo.db.testpymongo.find( {"Institute":"UCambridge", "Project.pname": "project 1" } )
Thank you !

You would need to use $elemMatch :
http://docs.mongodb.org/manual/reference/projection/elemMatch/
db.testpymongo.find( { "Project": { $elemMatch: { "pname": "project1" } } } )

How to Query this in MongoDB?

My items store in MongoDB like this :
{"ProductName":"XXXX",
"Catalogs" : [
{
"50008064" : "Apple"
},
{
"50010566" : "Box"
},
{
"50016422" : "Water"
}
]}
Now I want query all the items belong to Catalog:50008064,how to?
(the catalog id "50008064" , catalog name "Apple")

You cannot query this in an efficient manner and performance will decrease as your data grows. As such I would consider it a schema bug and you should refactor/migrate to the following model which does allow for indexing :
{"ProductName":"XXXX",
"Catalogs" : [
{
id : "50008064",
value : "Apple"
},
{
id : "50010566",
value : "Box"
},
{
id : "50016422",
value : "Water"
}
]}
And then index :
ensureIndex({'Catalogs.id':1})
Again, I strongly suggest you change your schema as this is a potential performance bottleneck you cannot fix any other way.

This should probably work according to the entry here, although this won't be very fast, as stated in in the link.
db.products.find({ "Catalogs.50008064" : { $exists: true } } )

using key as value in Mongoengine

I use mongoengine for mongodb in django.
but.. mongoengine fields (like StringField) makes me build up schema toward the way that I don't want. I mean, it strictly insists that I pre-write key name before I do know what it will be. for example...
in case that I do not know what key name will be put into database...
> for(var i=0; i<10; i++){
... o = {};
... o[i.toString()] = i + 100;
... db.test.save(o)
... }
> db.test.find()
{ "_id" : ObjectId("4ed623aa45c8729573313811"), "0" : 100 }
{ "_id" : ObjectId("4ed623aa45c8729573313812"), "1" : 101 }
{ "_id" : ObjectId("4ed623aa45c8729573313813"), "2" : 102 }
{ "_id" : ObjectId("4ed623aa45c8729573313814"), "3" : 103 }
{ "_id" : ObjectId("4ed623aa45c8729573313815"), "4" : 104 }
{ "_id" : ObjectId("4ed623aa45c8729573313816"), "5" : 105 }
{ "_id" : ObjectId("4ed623aa45c8729573313817"), "6" : 106 }
{ "_id" : ObjectId("4ed623aa45c8729573313818"), "7" : 107 }
{ "_id" : ObjectId("4ed623aa45c8729573313819"), "8" : 108 }
{ "_id" : ObjectId("4ed623aa45c872957331381a"), "9" : 109 }
[addition]
as you can see above, key is very different from each other..
just assume that "I do not know what key name will be put into document as key ahead of time
as dcrosta replied.. I am looking for a way to use mongoengine without specifying the fields ahead of time.
[/addition]
How can I do the same thing through mongoengine?
please give me schema design like
class Test(Document):
tag = StringField(db_field='xxxx')
[addition]
I don't know what 'xxxx' will be as key name.
sorry.. I'm Korean so my english is awkward.
please give me your some knowledge.
Thanks for reading this.
[/addition]

Have you considered using PyMongo directly instead of using Mongoengine? Mongoengine is designed to declare and validate a schema for your documents, and provides many tools and conveniences around that. If your documents are going to vary, I'm not sure Mongoengine is the right choice for you.
If, however, you have some fields in common across all documents, and then each document has some set of fields specific to itself, you can use Mongoengine's DictField. The downside of this is that the keys will not be "top-level", for instance:
class UserThings(Document):
# you can look this document up by username
username = StringField()
# you can store whatever you want here
things = DictField()
dcrosta_things = UserThings(username='dcrosta')
dcrosta_things.things['foo'] = 'bar'
dcrosta_things.things['bad'] = 'quack'
dcrosta_things.save()
Results in a MongoDB document like:
{ _id: ObjectId(...),
_types: ["UserThings"],
_cls: "UserThings",
username: "dcrosta",
things: {
foo: "bar",
baz: "quack"
}
}
Edit: I should also note, there's work in progress on the development branch of Mongoengine for "dynamic" documents, where attributes on the Python document instances will be saved when the model is saved. See https://github.com/hmarr/mongoengine/pull/112 for details and history.