Define DB and collection as 2 variables

[David_Lange]

I am missing something very fundamental to working within the shell.

The shell and server are 4.4.3. I don’t understand why I can declare a VAR for a DB and COLLECTION as one object and use it in a MQL. For example

MongoDB Enterprise PRIMARY> var col=db.product;
MongoDB Enterprise PRIMARY> col.find({}).count()
14027623

But I can’t define each component independently.

MongoDB Enterprise PRIMARY> var d=db
MongoDB Enterprise PRIMARY> var c=product
uncaught exception: ReferenceError: product is not defined :

Thanks
-Dave

[Stennie]

@David_Lange

var c=product
uncaught exception: ReferenceError: product is not defined :

Hi @David_Lange,

Per the error message, the problem is that product in this assignment context is expected to be a variable. You can use the typeof() method to check the type of a value:

> typeof(product)
undefined

> typeof('product')
string

If product happens to be a variable with a defined value, it can be used as the value for an assignment:

> var product = 'widgets'
> c=product
widgets

To assign the literal string product as a value, you need to use quotes:

var c = ‘product’

To fetch a collection using a variable, use db.getCollection().

> var c = 'product'
> db.getCollection(c).find()

For some general tips (and differences between scripted & interactive mongo), see: Write Scripts for the mongo Shell.

Regards,
Stennie