I need to store a recursive tree structure. A linked list.
So all the objects are the same. Each has a pointer to a parent object and each has an array of child objects.
Can I store such a structure in Mongo.
i.e. A Mongo collection of parent objects, each object holds within it a Mongo collection of child objects.
$a = $MyCollection->findOne(**some conditions)->Childs->find(...)
You cant store collections in collections. But you can store ids that reference objects in other collections. You would have to resolve the id to the document or element and then if that element stores more ids you would need to resolve those on and on. Documents are meant to be rich and duplicate data but in the docs they do explain that instead of embedding you can just use ids
MongoDB can store subdocuments:
Node
{
"value" : "root"
"children" : [ { "value" : "child1", "children" : [ ... ] },
{ "value" : "child2", "children" : [ ... ] } ]
}
However, I don't recommend to use subdocuments for tree structures or anything that is rather complex. Subdocuments are not first-level citizens; they are not collection items.
For instance, suppose you wanted to be able to quickly find the nodes with a given value. Through an index on value, that lookup would be fast. However, if the value is in a subdocument, it won't be indexed because it is not a collection element's value.
Therefore, it's usually better to do the serialization manually and store a list of ids instead:
Node
{
"_id" : ObjectId("..."),
"parentId" : ObjectId("..."), // or null, for root
}
You'll have to do some of the serialization manually to fetch the respective element's ids.
Hint
Suppose you want to fetch an entire branch of the tree. Instead of storing only the direct parent id, you can store all ancestor ids instead:
"ancestorIds": [id1, id2, id3]
Related
My MongoDB has a key-value pair structure, inside my document has a data field which is an array that contains many subdocuments of two fields: name and value.
How do I search for a subdocument e.g ( {"name":"position", "value":"manager"}) and also multiple (e.g. {"name":"age", "value" : {$ge: 30}})
EDIT: I am not looking for a specific subdocument as I mentioned in title (not positional reference), rather, I want to retrieve the entire document but I need it to match the two subdocuments exactly.
Here are 2 queries to find the following record:
{
"_id" : ObjectId("sometobjectID"),
"data" : [
{
"name" : "position",
"value" : "manager"
}
]
}
// Both value and name (in the same record):
db.demo.find({$elemMatch: {"value": "manager", "name":"position"}})
// Both value and name (not necessarily in the same record):
db.demo.find({"data.value": "manager", "data.name":"position"})
// Just value:
db.demo.find({"data.value": "manager"})
Note how the . is used, this works for all subdocuments, even if they are in an array.
You can use any operator you like here, including $gte
edit
$elemMatch added to answer because of #Veeram's response
This answer explains the difference between $elemMatch and .
How can I return a set of documents, each not containing a specific item in an inner array?
My data scheme is:
Posts:
{
"_id" : ObjectId("57f91ec96241783dac1e16fe"),
"votedBy" : [
{
"userId" : "101",
"vote": 1
},
{
"userId" : "202",
"vote": 2
}
],
"__v" : NumberInt(0)
}
I want to return a set of posts, non of which contain a given userId in any of the votedBy array items.
The official documentation implies that this is possible:
MongoDB documentation: Field with no specific array index
Though it returns an empty set (for the more simple case of finding a document with a specific array item).
It seems like I have to know the index for a correct set of results, like:
votedBy.0.userId.
This Question is the closest I found, with this solution (Applied on my scheme):
db.collection.find({"votedBy": { $not: {$elemMatch: {userId: 101 } } } })
It works fine if the only inner document in the array matches the one I wish not to return, but in the example case I specified above, the document returns, because it finds the userId=202 inner document.
Just to clarify: I want to return all the documents, that NONE of their votedBy array items have the given userId.
I also tried a simpler array, containing only the userId's as an array of Strings, but still, each of them receives an Id and the search process is just the same.
Another solution I tried is using a different collection for uservotes, and applying a lookup to perform a SQL-similar join, but it seems like there is an easier way.
I am using mongoose (node.js).
User $ne on the embedded userId:
db.collection.find({'votedBy.userId': {$ne: '101'}})
It will filter all the documents with at least one element of userId = "101"
I have documents stored like so:
{
...
"users" : [
{
"id" : "123456",
"username" : "John",
"address" : "fake st",
}
],
...
}
What is the best way of being able to retrieve all the documents with the username "john". Also, what are the proper ways of indexing this for performance considering its inside an array. Do I want to index "users", or is there a better way? This is inside a database with 50+ million documents.
To find all the documents which contain at least one "john" in the users array, use db.collection.find({"users.username":"john"});
To make this faster, create an index with db.collection.createIndex({"users.username":1});. Indexes in MongoDB can reach inside arrays and index individual array entries (this is called a multikey index).
Is it possible to point from one collection's item's value to another collection's item?
example:
db.col2.save( { value: 'test' } );
db.col1.save( { title: 'testing, something: [code to point to another collection's item] } );
db.col1.find().toArray()
[
{
"_id" : ObjectId([someobjectidhere]),
"title" : "testing",
"something": {
"value": "test"
}
}
]
Yes you can point to another document, however unlike SQL you can't do a join to retrieve both at the same time.
Therefore you would need to do 2 retrieves. One to get the first document (then extract the reference in code) and then use this reference to get the second document
MongoDB does not support joins. In MongoDB some data is “denormalized,” or stored with related data in documents to remove the need for joins. However, in some cases it makes sense to store related information in separate documents, typically in different collections or databases.
You can refer the doc for DBRef here
Suppose I have following collection :
{ _id" : ObjectId("4f1d8132595bb0e4830d15cc"),
"Data" : "[
{ "id1": "100002997235643", "from": {"name": "Joannah" ,"id": "100002997235643"} , "label" : "test" } ,
{ "id1": "100002997235644", "from": {"name": "Jon" ,"id": "100002997235644"} , "label" : "test1" }
]" ,
"stat" : "true"
}
How can I retrieve id1 , name , id ,label or any other element?
I am able to get _id field , DATA (complete array) but not the inner elements in DATA.
You cannot query for embedded structures. You always query for top level documents. If you want to query for individual elements from your array you will have to make those element top level documents (so, put them in their own collection) and maintain an array of _ids in this document.
That said, unless the array becomes very large it's almost always more efficient to simply grab your entire document and find the appropriate element in your app.
I don't think you can do that. It is explained here.
If you want to access specific fields, then following MongoDB Documentation,
you could add a flag parameter to your query, but you should redesign your documents for this to be useful:
Field Selection
In addition to the query expression, MongoDB queries can take some additional arguments. For example, it's possible to request only certain fields be returned. If we just wanted the social security numbers of users with the last name of 'Smith,' then from the shell we could issue this query:
// retrieve ssn field for documents where last_name == 'Smith':
db.users.find({last_name: 'Smith'}, {'ssn': 1});
// retrieve all fields *except* the thumbnail field, for all documents:
db.users.find({}, {thumbnail:0});