Query Data

Overview

Entity Framework Core allows you to work with data in your application without explicitly running database commands. To query your data, use the Language-Integrated Query (LINQ) syntax. LINQ allows you to write strongly typed queries using C#-specific keywords and operators. When you run the application, the EF Core Provider automatically translates the LINQ queries and runs them on the database using the MongoDB Query API.

In this guide you can see examples of common query operations on an application configured to use the EF Core Provider.

Find Entities

Find a single entity by using the FirstOrDefault() method, or find multiple entities by using the Where() method.

var planet = db.Planets.FirstOrDefault(p => p.name == "Mercury");
Console.WriteLine(planet.name);

var planets = db.Planets.Where(p => p.hasRings);
foreach (var p in planets)
{
    Console.WriteLine(p.name);
}

Sort Entities

Use the OrderBy() method to specify an order in which to return entities from a query. OrderBy() sorts the elements in ascending order based on a specified sort criteria.

The following code uses the OrderBy() method to find all planets and sort them by the value of the orderFromSun field in ascending order. It then prints the results to the console.

var planetList = db.Planets.OrderBy(p => p.orderFromSun);
foreach (var p in planetList)
{
   Console.WriteLine(p.name);
}

Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune

You can perform a secondary sort on your query by using the ThenBy() method. The ThenBy() method sorts the results of the OrderBy() method in ascending order based on a specified sort criteria. The ThenBy() method should be chained to the OrderBy() method.

The following code uses the OrderBy() and ThenBy() methods to find all planets and sort them by the hasRings() field, with a secondary sort on the name field.

var planetList = db.Planets.OrderBy(o => o.hasRings).ThenBy(o => o.name);
foreach (var p in planetList)
{
   Console.WriteLine("Has rings: " + p.hasRings + ", Name: " + p.name);
}

Has rings: False, Name: Earth
Has rings: False, Name: Mars
Has rings: False, Name: Mercury
Has rings: False, Name: Venus
Has rings: True, Name: Jupiter
Has rings: True, Name: Neptune
Has rings: True, Name: Saturn
Has rings: True, Name: Uranus

Scalar Aggregations

The EF Core Provider supports the following scalar aggregation methods:

• Count(): Returns the number of elements in a collection or the number of documents that match a predicate

• LongCount(): Returns the number of elements in a collection as a long or the number of documents that match a predicate

• Any(): Returns true if any elements in a collection match the predicate

• Max(): Returns the maximum value of a specified field in a collection

• Min(): Returns the minimum value of a specified field in a collection

• Sum(): Returns the sum of values of a specified field in a collection

• Average(): Returns the average of values of a specified field in a collection

The following sections show examples of each of the preceding methods.

var planetCount = db.Planets.Count();
Console.WriteLine("Planet Count: " + planetCount);

var planetCountWithRings = db.Planets.Count(p => p.hasRings);
Console.WriteLine("Planet Count with Rings: " + planetCountWithRings);

var planetCountLong = db.Planets.LongCount();
Console.WriteLine("Long Planet Count: " + longCount);

var planetCountLongWithRings = db.Planets.LongCount(p => p.hasRings);
Console.WriteLine("Long Planet Count with Rings: " + planetCountLongWithRings);

var results = db.Planets.Any(p => p.hasRings);
foreach (var p in results)
{
    Console.WriteLine("Planet with Rings: " + p.name);
}

var furthestPlanet = db.Planets.Max(p => p.orderFromSun);
Console.WriteLine("Furthest Planet: " + furthestPlanet.name);

var closestPlanet = db.Planets.Min(p => p.OrderFromSun);
Console.WriteLine("Closest Planet: " + closestPlanet.Name);

var totalMass = db.Planets.Sum(p => p.mass);
Console.WriteLine("Total Mass of Planets: " + totalMass);

var averageMass = db.Planets.Average(p => p.mass);
Console.WriteLine("Average Mass of Planets: " + averageMass);

Additional Information

To learn more about aggregations in MongoDB, see the Aggregation Operations guide in the MongoDB Server manual.

To learn more about the methods discussed in this guide, see the following .NET API documentation links:

• FirstOrDefault()

• Where()

• OrderBy()

• OrderByDescending()

• ThenBy()

• ThenByDescending()