# Other MongoDB hints and tips

## Example filter queries for target collections <a href="#mongodb-hints-filter-target-collections" id="mongodb-hints-filter-target-collections"></a>

When you configure subsetting in a MongoDB workspace, to filter a target collection, you can use either a percentage or a filter query.

For example, a customers collection contains the customer first name, last name, and membership status, as shown in the following sample records:

```json
{
	"FirstName": "John",
	"LastName": "Smith",
	"Member": true
}
{
	"FirstName": "Jane",
	"LastName": "Doe",
	"Member": false
}
```

To filter the records to only include members, the filter query would be:

```
{"Member":true}
```

As another example, the following filter query checks whether the eighth character in an object ID called `_id` is equal to 9.

```json
{
    $expr: {
        $eq: [
        { $substr: [ { $toString: "$_id" }, 8, 1 ] },
        "9"
        ]
        }
    }
}
```