How to limit maximum reference of the parental node in mongodb

Question

I have to create a tree structure with mongodb and of the four methods suggested by the official doc (parental reference, child reference, array of ancestors, materialized paths, nested sets) I have decided to use array of ancestors because the tier of the tree will be quite deep potentially thousands of levels or more. So I thought the penalty in storage could be compensated by faster speed due to less query being required when looking for all the parents of a node.

The problem is that the tree has to be strictly binary. So for example node 1 will only have node 2 and node 3 as its children and no more. Unfortunately only way I can think of doing this right now is to find all the documents that reference the specific parent that I would like to reference in the next document and if the count does not exceed 2, I insert the document referencing the target parent.

Obviously the problem is the multithreaded nature of mongodb and even if it wasn't, the request to look up the count that references the target parent could come in between the scanning and insertion of the previous document which would cause a third insertion if there were one document already referencing the target parent.

Is there a way I can make sure that only two nodes can reference a parent node in multithreaded environment of both mongodb and the server?

Answer 1

If you are using mongo version 3.6, you can enable schema validation using $jsonSchema at the server side

each document we update/insert will be validated against the validation schema, it the validation fails an error will be thrown and no modification will be made on the document

sample node collection schema

{
   _id : string,
   parent : [string, null],
   children : string[2]
}

validation schema

db.createCollection("node", {
   validator: {
      $jsonSchema: {
         bsonType: "object",
         required: [ "_id" ],
         properties: {
            parent: {
               bsonType: ["string", "null"],
               description: "must be a string"
            },
            children: {
               bsonType: ["array"],
               items : { bsonType: ["string"] },
               minItems: 0,
               maxItems: 2,
               description: "must be a array of string and max is 2"
            }
         }
      }
   }
});

inserts [with valid documents]

> db.node.insert( { _id: "Books", children: [ "Programming" ], parent: null } )
WriteResult({ "nInserted" : 1 })
> db.node.insert( { _id: "Programming", children: [ "Databases", "Languages" ], parent: "Books" } )
WriteResult({ "nInserted" : 1 })
> db.node.insert( { _id: "Languages", children: [ ], parent: "Programming" } )
WriteResult({ "nInserted" : 1 })
> db.node.insert( { _id: "Databases", children: [ "MongoDB", "dbm" ], parent: "Programming" } )
WriteResult({ "nInserted" : 1 })
> db.node.insert( { _id: "MongoDB", children: [ ], parent: "Databases" } )
WriteResult({ "nInserted" : 1 })
> db.node.insert( { _id: "dbm", children: [ ], parent: "Databases" } )
WriteResult({ "nInserted" : 1 })
> 

find

> db.node.find()
{ "_id" : "Books", "children" : [ "Programming" ], "parent" : null }
{ "_id" : "Programming", "children" : [ "Databases", "Languages" ], "parent" : "Books" }
{ "_id" : "Languages", "children" : [ ], "parent" : "Programming" }
{ "_id" : "Databases", "children" : [ "MongoDB", "dbm" ], "parent" : "Programming" }
{ "_id" : "MongoDB", "children" : [ ], "parent" : "Databases" }
{ "_id" : "dbm", "children" : [ ], "parent" : "Databases" }

insert with invalid document [children size > 2]

> db.node.insert({_id : "1", children : ["c1", "c2", "c3"], parent : "p1"})
WriteResult({
    "nInserted" : 0,
    "writeError" : {
        "code" : 121,
        "errmsg" : "Document failed validation"
    }
})
> 

insert failed with a validation error

update - trying to add 3rd child for _id Databases, failed with validation error

> db.node.updateOne( { _id: "Databases"}, {$push : {children: [ "Oracle" ]}} )
2018-02-25T21:00:08.087+0530 E QUERY    [thread1] WriteError: Document failed validation :
WriteError({
    "index" : 0,
    "code" : 121,
    "errmsg" : "Document failed validation",
    "op" : {
        "q" : {
            "_id" : "Databases"
        },
        "u" : {
            "$push" : {
                "children" : [
                    "Oracle"
                ]
            }
        },
        "multi" : false,
        "upsert" : false
    }
})
WriteError@src/mongo/shell/bulk_api.js:466:48
Bulk/mergeBatchResults@src/mongo/shell/bulk_api.js:846:49
Bulk/executeBatch@src/mongo/shell/bulk_api.js:910:13
Bulk/this.execute@src/mongo/shell/bulk_api.js:1154:21
DBCollection.prototype.updateOne@src/mongo/shell/crud_api.js:572:17
@(shell):1:1
> 

please refer schema-validation & jsonSchema for more options, adding validation to existing collection and handling validation failures