FAQ

Q. How to get parent object?

Supposedly we have: 

class Account 
{ 
  List<Phone> Phones; 
  List<Contact> Contacts; 
  //... 

} 

class Contact 
{ 
  List<Phone> Phones; 
  //... 

} 

class Phone 
{ 
  //some properties; 

} 

First, we query for a full list of Phone and display the result in a UI list. Next, once a user choose a particular Phone from UI, we need to retrieve its parent object (could be of type either Account of Contact, of course). 
What's the best way to achieve this?

A. Assume that phone is object from the database and being selected by user

SELECT Account WHERE Phones._items CONTAINS phone

or

SELECT Account WHERE Phones._items.number == 123456789

Where _items is an array inside List<T>. All such properties can be discovered with use of Reflector.

Q. Just wanted to check that I will not have an issue with multi-threaded concurrent access the Embedded DB.  All threads will be using the same connection.

A. Having a separate class with the public thread-safe method like Store-* or Delete-* with locking on the database operations looks like the safest solution, although not the most performing.

Please note that queries are lazy-evaluated, and running LINQ or SQL queries does not guarantee that the result set is on the client side. Because of this, you cannot query the database from different threads and getting the result set at the same time. If you need an immediate execution of queries, convert result sets into arrays of objects by calling .ToArray() method for IEnumerable. You might need the same locking approach for queries as for Store and Delete.

Querying the object returned by db.Query<T>() should provide some level of isolation as LINQ and SQL queries in the Eloquera DB cannot change data. The same reasoning on locking applies to such queries.

Q. I'm trying to store a class with a Tuple property and it looks like the properties of the tuple lose their values when fetching them back from DB.

A. Update. The Tuple objects now can be restored correctly starting with the version 3.1. The information below is retained for your convenience only:

It seems Tuple has all properties marked as read-only

public class Tuple<T1, T2, T3> : IStructuralEquatable,
IStructuralComparable, IComparable, ITuple
{
   // Fields
   private readonly T1 m_Item1;
   private readonly T2 m_Item2;
   private readonly T3 m_Item3;

Database saves the object, but object cannot be restored since fields are marked as "readonly".
One of the solutions is to create own tuple or remember objects that make this tuple e.g. T1, T2... and save these objects. Then use these objects to create tuple in runtime

Q. I installed the Server on a local pc with Win 7 Ultimate -32 bit and want to connect to it.
I get the error "The server has rejected the client credentials."

Connection string
     DB db = new DB("server=localhost;user=Admin;password=;options=none;");

Here is the section of the server config file. I played around with
AlllowGroups and AllowUsers - no success.

   <Server ServerPort="43962"
           Trace="true"
InMemoryAllowed="true"
           Secure="true"
AllowUsers=""
AllowGroups="Gäste;System;HomeUser;Everyone;Administrators;Administrator;Users;Guests;Administratoren;Benutzer;Jeder;" />

A. For the performance reasons, it is better to use Windows authentication using the current user account. For this, use the following connection string:
server=localhost;password=;options=none;

For debugging, remove all group names from the configuration file, and leave only Everyone (the group name may be different in the German version of Windows). Do not forget to restart the Eloquera DB Server service.

Q. Can Eloquera store objects of Hashtable type?

A. Eloquera Database can store any .NET objects, in general.

Although there are some types that are not stored. Those are pointer types (all types like byte* or IntPtr - they make no sense being restored later on), and instances of Type class (they cannot exist by themselves, they must be derived either by using typeof() operator or calling GetType() method on the instance).

But even if the class contains fields and properties of abovementioned types, it still can be stored in Eloquera DB on its own.

Q. I'm not sure if there already exist a method of authenticating with the DB server via a named user authentication rather than utilizing windows authentication, but it sure would be nice for a number of reasons. In other words a username and password that I make up, not a domain user:

Eloquera.config named user section example:

<NamedUser
username="sa"
password="saPassword"
access="read,write"
database="FinancialDB, RetailersDB, DistribDB" />

<NamedUser
username="jondoe"
password="jane"
access="read"
database="FinancialDB, RetailersDB, DistribDB" />

A. That's a good suggestion; we will consider it for one of our next releases. 

Currently, you can use Windows authentication without a set up domain, although it is not recommended because of poor connection performance. To enable it anyway, you need to turn off a simple sharing option and enable user and password authentication (if you are using a homegroup).

Q. I have exception with LINQ query

System.InvalidOperationException was unhandled by user code
Message=The operands for operator 'Equal' do not match the parameters of method 'op_Equality'.

Source=System.Core
StackTrace:
at System.Linq.Expressions.Expression.GetMethodBasedBinaryOperator(ExpressionType binaryType, Expression left, Expression right, MethodInfo method, Boolean liftToNull)
at System.Linq.Expressions.Expression.Equal(Expression left, Expression right, Boolean liftToNull, MethodInfo method)

Here is the query

from Dynamic d in db where d["ID"] == g select d

A. This is rather a LINQ feature. Cast doesn’t attempt to take any implicit cast operators (the LINQ Cast code is heavily optimized for special cases of all kinds, but nothing in that direction) into account. Use explicit cast in such cases.

from Dynamic d in db where (Guid)d["ID"] == g select d

Q. Is there a way to update an object in a disconnected scenario, like ajax or web services? In these scenarios the objects data will be coming in as json or xml, or some proprietary format, then deserialized into an object.

A. Starting from the version 3.0, the Eloquera DB supports Dynamic objects, which hold their state between database sessions. It means that the database can be closed after a Dynamic object is retrieved from the database, and opened again when we need to update the same Dynamic object without re-querying it. The same ‘disconnected’ technique applies to the native objects with a field with an [ID] attribute (starting from the version 3.1).

The native objects without an [ID] field can still be updated in the disconnected scenarios if their UID is known (by calling DB.GetID() method). The update is done by calling DB.Store(uid, obj) method.

The old approach to updating objects in the database is still working as well: object to be updated shall be retrieved from the database using their UID. Then required fields in the retrieved object shall be updated and the object can be stored back. This is the safest approach as it avoids any unintentional modification of objects’ UIDs.