https://stackoverflow.com/questions/48497487/write-a-translatable-method-for-linq-to-entities/

stack.48497487.1.cs

void Main()
{
	var ctx = new DemoContext();

	var inner1 = ctx.Filter<Bar>(value: "Const1");
	var inner2 = ctx.Filter<Bar>(value: "Const2");
	var inner3 = ctx.Filter<Bar>(value: "Const3");

	var query = from foo in ctx.Foos
				select new
				{
					P1 = inner1.FirstOrDefault(p => p.Y == foo.X1),
					P2 = inner2.FirstOrDefault(p => p.Y == foo.X2),
					P3 = inner3.FirstOrDefault(p => p.Y == foo.X3),
				};

	query.Dump();
}

public static class Extensions
{
	public static IQueryable<T> Filter<T>(this DemoContext instance, Expression<Func<T, bool>> predicate = null, string value = null)
		where T : class, IMarker
	{
		return instance
			.Set<T>()
			.Where(p => p.SomeProp == value)
			.Where(predicate);
	}
}

public class Foo : IEntity
{
	public string X1 { get; set; }
	public string X2 { get; set; }
	public string X3 { get; set; }

}

public class Bar : IMarker
{
	public string SomeProp { get; set; }
	public string Y { get; set; }
}

public interface IEntity
{
}

public interface IMarker : IEntity
{
	string SomeProp { get; set; }
}

public class DemoContext : DbContext
{
	public IDbSet<Foo> Foos { get; set; }
	public IDbSet<Bar> Bars { get; set; }
}

stack.48497487.cs

void Main()
{
	var ctx = new DemoContext();

	var query = from foo in ctx.Foos
				select new
				{
					P1 = ctx.Bars.FirstOrDefault(b => b.Y == foo.X1 && b.SomeProp == "Const1"),
					P2 = ctx.Find<Bar>(p => p.Y == foo.X2 && p.SomeProp == "Const2" ),
					P3 = ctx.Find<Bar>(p => p.Y == foo.X3, "Const3"),
				};
				
	query.Dump();
}

public static class Extensions
{
	public static T Find<T>(this DemoContext instance, Expression<Func<T, bool>> predicate)
		where T : class, IEntity
	{
		var query = instance
			.Set<T>()
			.Where(predicate)
			.FirstOrDefault();
		return query;
	}

	public static T Find<T>(this DemoContext instance, Expression<Func<T, bool>> predicate, string value)
		where T : class, IMarker
	{
		var query = instance
			.Set<T>()
			.Where(p => p.SomeProp == value)
			.Where(predicate)
			.FirstOrDefault();
		return query;
	}
}

public class Foo : IEntity
{
	public string X1 { get; set; }
	public string X2 { get; set; }
	public string X3 { get; set; }

}

public class Bar : IMarker
{
	public string SomeProp { get; set; }
	public string Y { get; set; }
}

public interface IEntity
{
}

public interface IMarker : IEntity
{
	string SomeProp { get; set; }
}

public class DemoContext : DbContext
{
	public IDbSet<Foo> Foos { get; set; }
	public IDbSet<Bar> Bars { get; set; }
}