Hướng dẫn findone mongodb

Starting in MongoDB 4.2, if the client that issued db.collection.findOne() disconnects before the operation completes, MongoDB marks db.collection.findOne() for termination using killOp.

Important

Language Consistency

The projection parameter determines which fields are returned in the matching documents. The projection parameter takes a document of the following form:

{ field1: , field2:  ... }

The following operation returns a single document from the bios collection:

The following operation returns the first matching document from the bios collection where either the field first in the embedded document name starts with the letter G or where the field birth is less than new Date('01/01/1945'):

db.bios.findOne(
   {
     $or: [
            { 'name.first' : /^G/ },
            { birth: { $lt: new Date('01/01/1945') } }
          ]
   }
)

The projection parameter specifies which fields to return. The parameter contains either include or exclude specifications, not both, unless the exclude is for the _id field.

db.bios.findOne(
    { },
    { name: 1, contribs: 1 }
)

db.bios.findOne(
   { contribs: 'OOP' },
   { _id: 0, 'name.first': 0, birth: 0 }
)

You cannot apply cursor methods to the result of findOne() because a single document is returned. You have access to the document directly:

var myDocument = db.bios.findOne();
if (myDocument) {
   var myName = myDocument.name;
   print (tojson(myName));
}

You can specify query options to modify query behavior and indicate how results are returned.

For example, to define variables that you can access elsewhere in the findOne method, use the let option. To filter results using a variable, you must access the variable within the $expr operator.

Create a collection cakeFlavors:

db.cakeFlavors.insertMany( [
   { _id: 1, flavor: "chocolate" },
   { _id: 2, flavor: "strawberry" },
   { _id: 3, flavor: "cherry" }
] )

The following example defines a targetFlavor variable in let and uses the variable to retrieve the chocolate cake flavor:

db.cakeFlavors.findOne(
   { $expr: { $eq: [ "$flavor", "$$targetFlavor" ] } },
   { _id: 0 },
   { let : { targetFlavor: "chocolate" }
} )

Output:

To see all available query options, see FindOptions.