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Code inspection: Possible multiple queries to the database (N+1 problem)

Last modified: 11 February 2024

Category: Entity Framework

ID: EntityFramework.NPlusOne.Usage

EditorConfig: resharper_entity_framework_n_plus_one_usage_highlighting

Default severity: Warning

Language: C#

Requires SWA: No

tip

You can suppress this inspection to ignore specific issues, change its severity level to make the issues less or more noticeable, or disable it altogether.

This inspection detects the N+1 Select problem, where unnecessary database calls are made when iterating a collection.

The problem can be illustrated with the following example, where Products and Details are 2 database tables and each Product entry is linked to multiple Detail entries.

// First DB query: get all 'Product' entries from the database
var products = dbContext.Products;

// Iterating 'Product' entries in memory
foreach (var product in products)
{
    // Accessing linked items with 'product.Details'
    // generates a new DB query for each 'Product',
    // which makes it N queries + the first query.
    foreach (var detail in product.Details)
    {
        // Do something
    }
}

To fix this problem, JetBrains Rider suggests using the Include feature of the Entity Framework Core, which will load all linked Detail entries right in the first query:

// Gets all 'Product' and all linked 'Detail' entries from the database
var products = dbContext.Products.Include(x => x.Details);

// Iterating 'Product' entries in memory
foreach (var product in products)
{
    // Accessing linked items with 'product.Details' in memory
    foreach (var detail in product.Details)
    {
        // Do something
    }
}

If the chain of navigational property calls is longer, the quick-fix will add ThenInclude as required:

// Gets all 'Product', all linked 'Detail', and all linked 'Line' entries from the database
var products = dbContext.Products.Include(x => x.Details).ThenInclude(x => x.Lines);
// Iterating 'Product' entries in memory
foreach (var product in products)
{
    // Accessing linked items with 'product.Details' in memory
    foreach (var detail in product.Details)
    {
        // Accessing linked items with 'detail.Lines' in memory
        foreach (var line in detail.Lines)
        {
            // Do something
        }
    }
}

note

Using multi-level Include/ThenInclude calls may lead to a cartesian explosion (depending on your database configuration). You may want to consider adding the AsSplitQuery() method to your query when using Entity Framework Core.

note

You can change the Entity Framework query analysis mode or disable all EF query inspections on the Editor | Inspection Settings | Entity Framework page of JetBrains Rider settings CtrlAlt0S.

See also

Concepts