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DOCSP-49771 - Add $elemMatch C# examples (#11912)
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‎content/manual/upcoming/source/includes/driver-examples/projection/ElemMatchExamples.cs

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using MongoDB.Bson;
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using MongoDB.Driver;
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namespace Projection;
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public class ElemMatchExamples
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{
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static string _uri = Environment.GetEnvironmentVariable("MONGODB_URI");
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static IMongoCollection<School> schoolsCollection = new MongoClient(_uri)
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.GetDatabase("example")
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.GetCollection<School>("schools");
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public static List<BsonDocument> ZipSearch()
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{
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// start zipSearch
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var results = schoolsCollection
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.Find(s => s.ZipCode == "63109")
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.Project(Builders<School>.Projection.ElemMatch(
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field: school => school.Students,
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filter: student => student.School == 102
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)
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).ToList();
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// end zipSearch
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return results;
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}
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public static List<BsonDocument> ZipMultipleSearch()
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{
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// start zipMultipleSearch
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var results = schoolsCollection
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.Find(s => s.ZipCode == "63109")
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.Project(Builders<School>.Projection.ElemMatch(
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field: school => school.Students,
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filter: student => (student.School == 102) && (student.Age > 10)
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)
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).ToList();
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// end zipMultipleSearch
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return results;
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}
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public static List<BsonDocument> ZipAthleticsIncomplete()
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{
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// start zipAthleticsIncomplete
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var results = schoolsCollection
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.Find(s => s.ZipCode == "63109")
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.Project(Builders<School>.Projection.ElemMatch(
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"athletics",
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Builders<School>.Filter.Eq("athletics", "basketball"))
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).ToList();
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// end zipAthleticsIncomplete
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return results;
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}
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public static List<BsonDocument> ZipAthletics()
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{
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// start zipAthletics
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var results = schoolsCollection
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.Find(s => s.ZipCode == "63109")
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.Project(Builders<School>.Projection.ElemMatch(
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field: "athletics",
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filter: Builders<School>.Filter.Eq("$eq", "basketball"))
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).ToList();
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// end zipAthletics
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return results;
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}
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}

‎content/manual/upcoming/source/includes/driver-examples/projection/School.cs

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public class School
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{
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public string Id { get; set; }
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[BsonElement("zipcode")]
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public string ZipCode { get; set; }
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public Student[] Students { get; set; }
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public string[] Athletics { get; set; }
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}

‎content/manual/upcoming/source/includes/driver-examples/projection/Student.cs

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public class Student
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{
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public string Id { get; set; }
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public string Name { get; set; }
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public int School { get; set; }
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public int Age { get; set; }
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}

‎content/manual/upcoming/source/reference/operator/projection/elemMatch.txt

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- You cannot specify a :query:`$text` query operator in an
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:projection:`$elemMatch`.
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Examples
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--------
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.. code-block:: javascript
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:copyable: true
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db.school.insertMany( [
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{
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{
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_id: 1,
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zipcode: "63109",
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students: [
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{ name: "ruth", school: 102, age: 16 },
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]
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}
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] )
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.. tabs-drivers::
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.. tab:: MongoDB Shell
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:tabid: shell
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.. tab:: C#
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:tabid: csharp
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You can model these documents by using the following C# classes:
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.. literalinclude:: /includes/driver-examples/projection/School.cs
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:language: csharp
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.. literalinclude:: /includes/driver-examples/projection/Student.cs
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:language: csharp
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Zipcode Search
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~~~~~~~~~~~~~~
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The following :method:`~db.collection.find()` operation
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queries for all documents where the value of the ``zipcode``
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field is ``63109``. The :projection:`$elemMatch` projection
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returns only the **first** matching element of the ``students``
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array where the ``school`` field has a value of ``102``:
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.. tabs-selector:: drivers
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.. code-block:: javascript
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:copyable: true
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.. tabs-drivers::
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.. tab:: MongoDB Shell
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:tabid: shell
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db.schools.find( { zipcode: "63109" },
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{ students: { $elemMatch: { school: 102 } } } )
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The following :method:`~db.collection.find()` operation
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queries for all documents where the value of the ``zipcode``
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field is ``"63109"``. The :projection:`$elemMatch` projection
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returns only the **first** matching element of the ``students``
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array where the ``school`` field has a value of ``102``:
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The operation returns the following documents that have ``zipcode``
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equal to ``63109`` and projects the ``students`` array using
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.. code-block:: javascript
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:copyable: true
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db.schools.find( { zipcode: "63109" },
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{ students: { $elemMatch: { school: 102 } } } )
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.. tab:: C#
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:tabid: csharp
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To perform an ``$elemMatch`` projection when using the .NET/C# driver, call the
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``ElemMatch()`` method on the projection builder. Pass the name of the array field
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to project and the filter to apply to the array elements.
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The following code example finds all documents in which the value of the ``Zipcode``
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field is ``"63109"``. For each matching document, the projection returns the
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following fields:
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- ``Id``
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- The first element of the ``Students`` array in which the value of the nested
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``School`` field has the value ``102``
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.. literalinclude:: /includes/driver-examples/projection/ElemMatchExamples.cs
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:language: csharp
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:start-after: // start zipSearch
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:end-before: // end zipSearch
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:dedent: 8
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The operation returns the following documents that have a ``zipcode`` value of
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``"63109"`` and projects the ``students`` array using
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:projection:`$elemMatch`:
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.. code-block:: javascript
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:copyable: false
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{ "_id" : 1, "students" : [ { "name" : "john", "school" : 102, "age" : 10 } ] }
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{ "_id" : 3 }
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The :projection:`$elemMatch` projection can specify criteria on multiple
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fields:
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fields.
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The following :method:`~db.collection.find()` operation
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queries for all documents where the value of the ``zipcode``
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field is ``63109``. The projection includes the **first**
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matching element of the ``students`` array where the ``school``
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field has a value of ``102`` **and** the ``age`` field is greater
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than ``10``:
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.. tabs-drivers::
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.. code-block:: javascript
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:copyable: true
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.. tab:: MongoDB Shell
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:tabid: shell
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The following :method:`~db.collection.find()` operation
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queries for all documents where the value of the ``zipcode``
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field is ``"63109"``. The projection includes the **first**
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matching element of the ``students`` array where the ``school``
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field has a value of ``102`` **and** the ``age`` field is greater
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than ``10``:
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db.schools.find( { zipcode: "63109" },
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{ students: { $elemMatch: { school: 102, age: { $gt: 10} } } } )
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.. code-block:: javascript
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:copyable: true
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The operation returns the three documents that have ``zipcode`` equal to ``63109``:
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db.schools.find( { zipcode: "63109" },
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{ students: { $elemMatch: { school: 102, age: { $gt: 10} } } } )
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.. tab:: C#
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:tabid: csharp
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The following code example finds all documents in which the value of the ``Zipcode``
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field is ``"63109"``. For each matching document, the projection returns the following fields:
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- ``Id``
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- The first element of the ``Students`` array in which the value of the nested
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``School`` field has the value ``102`` and the ``Age`` field has a value greater than
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``10``
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.. literalinclude:: /includes/driver-examples/projection/ElemMatchExamples.cs
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:language: csharp
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:start-after: // start zipMultipleSearch
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:end-before: // end zipMultipleSearch
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:dedent: 8
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The operation returns the three documents that have a ``zipcode`` value of ``"63109"``:
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.. code-block:: javascript
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:copyable: false
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{ "_id" : 1, "students" : [ { "name" : "jess", "school" : 102, "age" : 11 } ] }
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{ "_id" : 3 }
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since no array element matched the :projection:`$elemMatch` criteria.
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The argument to :projection:`$elemMatch` matches elements of the array that
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``$elemMatch`` is projecting. If you specify an equality with a field
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``$elemMatch`` is projecting. If you specify an equality with a field
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name to ``$elemMatch``, it attempts to match objects within the array.
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For example, ``$elemMatch`` attempts to match objects, instead of scalar
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values, within the array for the following in the projection:
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.. code-block:: javascript
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:copyable: true
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.. tabs-drivers::
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db.schools.find( { zipcode: "63109" },
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{ athletics: { $elemMatch: { athletics: "basketball" } } })
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.. tab:: MongoDB Shell
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:tabid: shell
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To match scalar values, use the equality operator along with the scalar
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value that you want to match (``{$eq: <scalar value>}``). For example,
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the following :method:`~db.collection.find()` operation queries for all
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documents where the value of the ``zipcode`` field is ``63109``. The
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projection includes the matching element of the ``athletics`` array
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where the value is ``basketball``:
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.. code-block:: javascript
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.. code-block:: javascript
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:copyable: true
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db.schools.find( { zipcode: "63109" },
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{ athletics: { $elemMatch: { athletics: "basketball" } } })
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.. tab:: C#
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:tabid: csharp
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.. literalinclude:: /includes/driver-examples/projection/ElemMatchExamples.cs
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:language: csharp
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:start-after: // start zipAthleticsIncomplete
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:end-before: // end zipAthleticsIncomplete
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:dedent: 8
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The preceding examples return the documents that have a ``zipcode`` value of
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``"63109"``, but these documents include only the ``_id`` field because the projection
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operation found no matching elements.
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.. tabs-drivers::
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.. tab:: MongoDB Shell
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:tabid: shell
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To match scalar values, use the equality operator along with the scalar
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value that you want to match (``{$eq: <scalar value>}``). For example,
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the following :method:`~db.collection.find()` operation queries for all
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documents where the value of the ``zipcode`` field is ``"63109"``. The
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projection includes the matching element of the ``athletics`` array
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where the value is ``basketball``:
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.. code-block:: javascript
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:copyable: true
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db.schools.find( { zipcode: "63109" },
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{ athletics: { $elemMatch: { $eq: "basketball" } } })
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.. tab:: C#
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:tabid: csharp
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To perform an ``$elemMatch`` operation against scalar values in an array when using
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the .NET/C# driver, call the ``ElemMatch()`` method
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on the projection builder. Pass the name of the array field to project and an
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equality filter for the field ``"$eq"`` and the value you want to compare against.
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db.schools.find( { zipcode: "63109" },
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{ athletics: { $elemMatch: { $eq: "basketball" } } })
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.. literalinclude:: /includes/driver-examples/projection/ElemMatchExamples.cs
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:language: csharp
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:start-after: // start zipAthletics
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:end-before: // end zipAthletics
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:dedent: 8
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The operation returns the three documents that have ``zipcode`` equal to
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``63109``:
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The operation returns the three documents that have ``zipcode`` value of
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``"63109"``. The returned documents include the ``_id`` field and matching
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elements of the ``athletics`` array, if any.
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.. code-block:: javascript
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:copyable: false
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[
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{ _id : 1 },
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{ _id: 1 },
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{ _id: 3, athletics: [ 'basketball' ] },
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{ _id : 4 }
300+
{ _id: 4 }
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]
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The document with ``_id`` equal to ``3`` is the only document that
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matched the :projection:`$elemMatch` criteria.
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matched the :projection:`$elemMatch` criteria.
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.. seealso::
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