dot notation¶
Estimated time to read: 2 minutes
To access nested documents within an array field, MQL makes use of dot-notation.
Fields within a sub-document can be accessed
The field name from the sub-document, follows the top-level field separated by a dor and then wrapped in quotes. It's is essentially a path to the field to be queried within the array within the document.
For example to find all instances of "type" : "Point" within the start station location field, the following query could be used:
db.trips.findOne( {"station station location.type": "Point"} )
dot-notation can be used to traverse deep objects to get the last atomic non-document value in the hierachy.
Example¶
The query below looks at the first element in the relationships array using dot-notation. Array elements in most languages and data structures are enumerated staring from zero.
Example Breakdown¶
0: position of the first array element person: field name with a nested object as a value last_name: field name with the "person" sub-document "Zuckerberg": value to be searched for { "name": 1 }: projection to only include the company name in the resulting cursor
Further Examples¶
CEOs¶
Find the count of documents where, CEOs named Mark listed first in the relationships array.
db.companies.find({ "relationships.0.person.first_name": "Mark",
"relationships.0.title": { "$regex": "CEO" } },
{ "name": 1 }).count()
Return the documents of the above example, in a pretty format!
db.companies.find({ "relationships.0.person.first_name": "Mark",
"relationships.0.title": {"$regex": "CEO" } },
{ "name": 1 }).pretty()
Past CEOs¶
All senior executives named Mark listed in the relationships array who no longer work at their company.
To do this, the following queries shall be checked. { "is_past: true" } and { "person.first_name: "Mark" }. This can be done using an $elemMatch operator to look through every array element and match the conditions.