如何在MongoDB C#驱动中使用投影？含忽略_id的代码实现

Hey there! Let's tackle your two MongoDB C# driver projection questions step by step with practical code examples. I'll use a simple Product entity class for consistency across examples:

public class Product
{
    public ObjectId Id { get; set; }
    public string Name { get; set; }
    public decimal Price { get; set; }
    public string Category { get; set; }
}

Projection lets you fetch only specific fields from your documents instead of the entire document, which saves bandwidth and improves performance. There are two common approaches to implement this:

Using the Builders<T>.Projection API #

This method gives you fine-grained control over which fields to include or exclude. Here's an example where we only retrieve the Name and Price fields from products in the "Electronics" category:

var collection = mongoDatabase.GetCollection<Product>("products");

// Define the projection: include Name and Price
var projection = Builders<Product>.Projection
    .Include(p => p.Name)
    .Include(p => p.Price);

// Execute the query with projection
var results = await collection.Find(p => p.Category == "Electronics")
    .Project(projection)
    .ToListAsync();

Using Projection Expressions (Concise Syntax) #

For simpler scenarios, you can use a lambda expression directly in the Project() method to define the fields you want. This will automatically create a projection that only includes the specified fields:

var results = await collection.Find(p => p.Category == "Electronics")
    .Project(p => new { p.Name, p.Price }) // Project to an anonymous type
    .ToListAsync();

You can also project to a custom DTO (Data Transfer Object) if you want a strongly-typed result instead of an anonymous type:

public class ProductSummary
{
    public string Name { get; set; }
    public decimal Price { get; set; }
}

var results = await collection.Find(p => p.Category == "Electronics")
    .Project<ProductSummary>(p => new ProductSummary { Name = p.Name, Price = p.Price })
    .ToListAsync();

By default, MongoDB returns the _id field even if you don't explicitly include it in your projection. To exclude it, you have a couple of options depending on which projection approach you're using:

With Builders<T>.Projection API #

Use the Exclude() method to explicitly exclude the _id field. Here's an example where we fetch Name and Category without the _id:

var projection = Builders<Product>.Projection
    .Include(p => p.Name)
    .Include(p => p.Category)
    .Exclude(p => p.Id); // Exclude the _id field

var results = await collection.Find(_ => true)
    .Project(projection)
    .ToListAsync();

With Projection Expressions #

When using a projection expression (to anonymous type or DTO), you just need to not include the Id property in your projection. The MongoDB driver will automatically exclude _id in this case:

// Project to anonymous type (no Id, so _id is excluded)
var results = await collection.Find(_ => true)
    .Project(p => new { p.Name, p.Category })
    .ToListAsync();

// Or project to a DTO that doesn't have an Id property
public class ProductCategoryView
{
    public string Name { get; set; }
    public string Category { get; set; }
}

var results = await collection.Find(_ => true)
    .Project<ProductCategoryView>(p => new ProductCategoryView { Name = p.Name, Category = p.Category })
    .ToListAsync();

Note: #

If you're using projection expressions and do include the Id property, it will be mapped to the _id field as expected. Only when you omit it does the driver exclude _id from the result.

内容的提问来源于stack exchange，提问作者Guru Ganesh