[BUG] Find all related orders of a customer returns empty

[kfirpeled]

Version
LiteDb 5.0.7
OSX Catalina 10.15.4
dotnet 3.1.102

Describe the bug
Finding customer's order returns zero results when using lambda expression.
Digging into this deep I could see that the BsonMapper is not resolving the expression correctly

Code to Reproduce

    public class Order : BaseEntity
    {
        public Customer? Customer { get; set; }
    }

    public class Customer : BaseEntity
    {
        public string? Name { get; set; }
    }

    public class BaseEntity
    {
        public Guid Id { get; set; }
    }
    public class CustomerRelationshipsTests : IDisposable
    {
        private LiteDatabase _database;
        private ILiteCollection<Customer> _customerCollection;
        private ILiteCollection<Order> _orderCollection;

        public CustomerRelationshipsTests()
        {
            BsonMapper.Global.Entity<Order>().DbRef(order => order.Customer);

            _database = new LiteDatabase("customer1.db");

            _orderCollection = _database.GetCollection<Order>();
            _customerCollection = _database.GetCollection<Customer>();
        }


        [Fact]
        public void Find_ByRelationId_Success()
        {
            var customer = new Customer() {Name = "John",};

            Assert.True(_customerCollection.Upsert(customer));
            Assert.True(_customerCollection.Upsert(new Customer() {Name = "Anonymous"}));

            Assert.NotEqual(Guid.Empty, customer.Id);

            var order = new Order()
            {
                Customer = customer,
            };
            var order2 = new Order()
            {
                Customer = new Customer() {Id = customer.Id},
            };
            var orphanOrder = new Order();

            Assert.True(_orderCollection.Upsert(orphanOrder));
            Assert.True(_orderCollection.Upsert(order));
            Assert.True(_orderCollection.Upsert(order2));

            customer.Name = "Josh";
            Assert.True(_customerCollection.Update(customer));


            var actualOrders = _orderCollection
                .Include(orderEntity => orderEntity.Customer)
                // NOT WORKING (BsonExpression generated from the lambda is $.Customer._id instead of $.Customer.$id)
                .Find(orderEntity => orderEntity.Customer.Id == customer.Id)
                // .Find(Query.EQ("$.Customer.$id", customer.Id)) WORKS - returning results correctly
                // .Find(Query.EQ("$.Customer._id", customer.Id)) NOT WORKING
                .ToList();

            Assert.Equal(2, actualOrders.Count); // Fails here, actual result is zero
            Assert.Equal(new[] {customer.Name, customer.Name},
                actualOrders.Select(actualOrder => actualOrder.Customer.Name));
            Assert.Equal(2, (_customerCollection.FindAll().ToList()).Count);
            Assert.Equal(3, (_orderCollection.FindAll().ToList()).Count);
        }

        public void Dispose()
        {
            _database.DropCollection(_orderCollection.Name);
            _database.DropCollection(_customerCollection.Name);
            _database.Dispose();
        }
    }

**Is it related to BsonMapper ? **

var expr = BsonMapper.Global.GetExpression<Order, bool>(orderEntity => orderEntity.Customer.Id == customer.Id);

Returns ($.Customer._id=@p0)
Should it return ($.Customer.$id=@p0) ? Or the issue is somewhere else?

Expected behavior
I expect to use find with lambda to select customer's order by the customer's id.
A workaround for me is to build the query myself (which is not ideal)

[kfirpeled]

Update:
It seems that since 5.0.7 this is reproduced constantly,
In previous versions, it reproduced sporadically 🤷🏼‍♀️

[lbnascimento]

@kfirp-bs This issue has been fixed in the latest master and its fix will be present in the next incremental release.