Query filter

This scenario demonstrate how to get rows from database with Entity Framework Core's query filter feature. This is for Multi-Tenancy scenarios.

Scenario Prototype

public interface IQueryFilterScenario<TStudent>
    where TStudent : class, IStudent, new()
{
    IList<TStudent> GetStudents(int schoolId);
}

Entity Framework Core

A query filter is setup in the OnModelCreating event of the DB Context.

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    if (modelBuilder == null)
        throw new ArgumentNullException(nameof(modelBuilder), $"{nameof(modelBuilder)} is null.");

    base.OnModelCreating(modelBuilder);

    //The ISchool interface is applied to all entities that need to be filtered by tenant.
    modelBuilder.SetQueryFilterOnAllEntities<ISchool>(s => s.SchoolId == SchoolId);
}

Normally this would be done for each table. But by placing a marker interface (ISchool in this case) and some helper methods, it can be automatically applied to all tables.

/*
Copyright Phil Haack
Licensed under the MIT license - https://github.com/haacked/CodeHaacks/blob/main/LICENSE.
*/

using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Metadata.Builders;
using Microsoft.EntityFrameworkCore.Query;
using System.Linq.Expressions;
using System.Reflection;

namespace Recipes.EntityFrameworkCore.QueryFilter.Helpers;

public static class ModelBuilderExtensions
{
    static readonly MethodInfo SetQueryFilterMethod = typeof(ModelBuilderExtensions)
            .GetMethods(BindingFlags.NonPublic | BindingFlags.Static)
            .Single(t => t.IsGenericMethod && t.Name == nameof(SetQueryFilter));

    public static void SetQueryFilterOnAllEntities<TEntityInterface>(
        this ModelBuilder builder,
        Expression<Func<TEntityInterface, bool>> filterExpression)
    {
        foreach (var type in builder.Model.GetEntityTypes()
            .Where(t => t.BaseType == null)
            .Select(t => t.ClrType)
            .Where(t => typeof(TEntityInterface).IsAssignableFrom(t)))
        {
            builder.SetEntityQueryFilter(
                type,
                filterExpression);
        }
    }

    static void SetEntityQueryFilter<TEntityInterface>(
        this ModelBuilder builder,
        Type entityType,
        Expression<Func<TEntityInterface, bool>> filterExpression)
    {
        SetQueryFilterMethod
            .MakeGenericMethod(entityType, typeof(TEntityInterface))
           .Invoke(null, new object[] { builder, filterExpression });
    }

    static void SetQueryFilter<TEntity, TEntityInterface>(
        this ModelBuilder builder,
        Expression<Func<TEntityInterface, bool>> filterExpression)
            where TEntityInterface : class
            where TEntity : class, TEntityInterface
    {
        var concreteExpression = filterExpression
            .Convert<TEntityInterface, TEntity>();
        builder.Entity<TEntity>()
            .AppendQueryFilter(concreteExpression);
    }

    // CREDIT: This comment by magiak on GitHub https://github.com/dotnet/efcore/issues/10275#issuecomment-785916356
    static void AppendQueryFilter<T>(this EntityTypeBuilder entityTypeBuilder, Expression<Func<T, bool>> expression)
        where T : class
    {
        var parameterType = Expression.Parameter(entityTypeBuilder.Metadata.ClrType);

        var expressionFilter = ReplacingExpressionVisitor.Replace(
            expression.Parameters.Single(), parameterType, expression.Body);

        var currentQueryFilter = entityTypeBuilder.Metadata.GetQueryFilter();
        if (currentQueryFilter != null)
        {
            var currentExpressionFilter = ReplacingExpressionVisitor.Replace(
                currentQueryFilter.Parameters.Single(), parameterType, currentQueryFilter.Body);
            expressionFilter = Expression.AndAlso(currentExpressionFilter, expressionFilter);
        }

        var lambdaExpression = Expression.Lambda(expressionFilter, parameterType);
        entityTypeBuilder.HasQueryFilter(lambdaExpression);
    }
}

// Copyright (c) 2020 Phil Haack, GitHub: haacked
// https://gist.github.com/haacked/febe9e88354fb2f4a4eb11ba88d64c24

using System.Collections.ObjectModel;
using System.Linq.Expressions;

namespace Recipes.EntityFrameworkCore.QueryFilter.Helpers;

public static class ExpressionExtensions
{
    // This magic is courtesy of this StackOverflow post.
    // https://stackoverflow.com/questions/38316519/replace-parameter-type-in-lambda-expression
    // I made some tweaks to adapt it to our needs - @haacked
    public static Expression<Func<TTarget, bool>> Convert<TSource, TTarget>(
        this Expression<Func<TSource, bool>> root)
    {
        var visitor = new ParameterTypeVisitor<TSource, TTarget>();
        return (Expression<Func<TTarget, bool>>)visitor.Visit(root);
    }

    class ParameterTypeVisitor<TSource, TTarget> : ExpressionVisitor
    {
        private ReadOnlyCollection<ParameterExpression>? _parameters;

        protected override Expression VisitParameter(ParameterExpression node)
        {
            return _parameters?.FirstOrDefault(p => p.Name == node.Name)
                   ?? (node.Type == typeof(TSource) ? Expression.Parameter(typeof(TTarget), node.Name) : node);
        }

        protected override Expression VisitLambda<T>(Expression<T> node)
        {
            _parameters = VisitAndConvert(node.Parameters, "VisitLambda");
            return Expression.Lambda(Visit(node.Body), _parameters);
        }
    }
}

This example shows it being used.

public class QueryFilterScenario : IQueryFilterScenario<Student>
{
    private readonly Func<int, OrmCookbookContextWithQueryFilter> CreateFilteredDbContext;

    public QueryFilterScenario(Func<int, OrmCookbookContextWithQueryFilter> dBContextFactory)
    {
        CreateFilteredDbContext = dBContextFactory;
    }

    public IList<Student> GetStudents(int schoolId)
    {
        using (var context = CreateFilteredDbContext(schoolId))
        {
            //SchoolId filter is automatically applied
            return context.Students.OrderBy(s => s.Name).ToList();
        }
    }
}