Expression Trees

Definition

Expression Trees allow building queries or filters at runtime as a syntax tree, instead of writing them statically in source code. EF Core translates these trees into SQL.

In Granit, ApplyGranitConventions() dynamically builds EF Core query filters for each entity by combining ISoftDeletable, IActive, and IMultiTenant filters into a single expression.

Diagram

flowchart TD
    AGC["ApplyGranitConventions()"] --> SCAN[Scan DbContext entities]
    SCAN --> CHECK{Which interfaces implemented?}

    CHECK -->|ISoftDeletable| E1["Expression: !e.IsDeleted<br/>or !proxy.SoftDeleteEnabled"]
    CHECK -->|IActive| E2["Expression: e.IsActive<br/>or !proxy.ActiveEnabled"]
    CHECK -->|IMultiTenant| E3["Expression: e.TenantId == proxy.CurrentTenantId<br/>or !proxy.MultiTenantEnabled"]

    E1 --> COMBINE["Expression.AndAlso()<br/>Combine all conditions"]
    E2 --> COMBINE
    E3 --> COMBINE

    COMBINE --> LAMBDA["Expression.Lambda of Func T bool"]
    LAMBDA --> HQF["entity.HasQueryFilter(lambda)"]

Implementation in Granit

Component	File	Lines
ApplyGranitConventions()	src/Granit.Persistence/Extensions/ModelBuilderExtensions.cs	54-126
FilterProxy	src/Granit.Persistence/Extensions/ModelBuilderExtensions.cs	133-140

Why a FilterProxy?

EF Core cannot translate arbitrary method calls (like dataFilter.IsEnabled<ISoftDeletable>()) in a query filter. The FilterProxy exposes simple properties that EF Core extracts as SQL parameters:

// FilterProxy exposes properties that EF Core understands
internal sealed class FilterProxy(IDataFilter? dataFilter, ICurrentTenant? tenant)
{
    public bool SoftDeleteEnabled => dataFilter?.IsEnabled<ISoftDeletable>() ?? true;
    public bool ActiveEnabled => dataFilter?.IsEnabled<IActive>() ?? true;
    public bool MultiTenantEnabled => dataFilter?.IsEnabled<IMultiTenant>() ?? true;
    public Guid? CurrentTenantId => tenant?.Id;
}

Why a single HasQueryFilter?

EF Core (versions before 10) overwrites previous query filters if HasQueryFilter() is called multiple times on the same entity. The combined expression via AndAlso solves this problem in a single call.

Rationale

Dynamic construction handles all combinations of interfaces automatically (an entity can implement 0, 1, 2, or 3 marker interfaces) without writing specific code for each combination (2^3 = 8 cases).

Usage example

// The application calls a single line in OnModelCreating
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.ApplyGranitConventions(serviceProvider);
    // -> Dynamically builds query filters for all entities
}

// The filter is automatic and transparent
List<Patient> patients = await db.Patients.ToListAsync(ct);
// Generated SQL:
// SELECT * FROM Patients
// WHERE (@SoftDeleteEnabled = 0 OR IsDeleted = 0)
//   AND (@MultiTenantEnabled = 0 OR TenantId = @CurrentTenantId)

Further reading

• Expression Trees — Microsoft .NET Documentation