MongoDB Links installation in the database

Preamble

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External keys may be set in relational databases when fields from one table refer to fields in another table. You can also set links in MongoDB.

Manual link setting

The manual setting of references comes down to assigning the value of field _id of one document to the field of another document. Let’s say we may have collections representing companies and employees working in these companies. So, let us first add a document representing the company to the collection of companies:

db.companies.insert({"_id" : "microsoft", year: 1974})

Now let’s add a document representing the employee to the collection. In this document, there will be a company field representing the company where the employee works. And it is very important that we set the value of the _id key of the document added above, rather than the company object, as the value for this field:

db.users.insert({name: "Tom", age: 28, company: "microsoft"})

Now we get the document from the user’s collection:

user = db.users.findOne()

In this case, it means that the above-added element will be the only one in the collection.

After that, the console outputs the resulting document. And now we will find a link to his company in the collection of companies:

db.companies.findOne({_id: user.company})

And if a document with such identifier is detected, it is displayed on the console:

Automatic binding

Using DBRef functionality, we can set automatic linking between documents. Let’s have a look at an example of how this functionality can be applied. First, let’s add a new document to the collection of companies:

apple=({"name" : "apple", "year": 1976})
> db.companies.save(apple)

Note that in this case the saving is done using the same method, not insert. The save method generates _id when adding a new document. And after saving we can display the document in the console: > apple

Now let’s create a new document for the person collection, where the company key will bind to the newly added apple document:

steve = ({"name": "Steve", "age": 25, company: new DBRef('companies', apple._id)})
> db.users.save(steve)

And we can test it:

db.companies.findOne({_id: steve.company.$id})

Having looked at the example, now we will analyze the organization of links between documents. The following company expression was used to link to an apple document: new DBRef(‘companies’, apple._id)}). The formal DBRef syntax is as follows:

{ "$ref" : name_collection, "$id" : value [, "$db" : name_bd ]}

The first parameter $ref points to the collection where the linked document is stored. The second parameter points to a value that will represent something like an external key. The third optional parameter points to a database.

During testing, the expression _id: steve.company.$id shall be used as a sample request. Since person.company now represents the object new DBRef(‘companies’, apple._id)}), we need to specify the steve.company.$id parameter.

MongoDB Links Tutorial – Connecting to MongoDB

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