/// <summary>
/// Based on an expression, returns document field mapped from class Property.
/// Support multi level dotted notation: x => x.Customer.Name
/// Prefix is used on array expression like: x => x.Customers.Any(z => z.Name == "John") (prefix = "Customers."
/// </summary>
public string GetField(Expression expr, string prefix = "")
{
var property = prefix + expr.GetPath();
var parts = property.Split('.');
var fields = new string[parts.Length];
var type = _type;
var isdbref = false;
// loop "first.second.last"
for (var i = 0; i < parts.Length; i++)
{
var entity = _mapper.GetEntityMapper(type);
var part = parts[i];
var prop = entity.Members.Find(x => x.MemberName == part);
if (prop == null)
{
throw LiteException.PropertyNotMapped(property);
}
// if property is a IEnumerable, gets underlayer type (otherwise, gets PropertyType)
type = prop.UnderlyingType;
fields[i] = prop.FieldName;
if (prop.FieldName == "_id" && isdbref)
{
isdbref = false;
fields[i] = "$id";
}
// if this property is DbRef, so if next property is _id, change to $id
if (prop.IsDbRef)
{
isdbref = true;
}
}
return(string.Join(".", fields));
}