JPA tips 2

A previous post about this topic: http://hilaltarakci.blogspot.com/2011/03/jpa-tips.html

Actually, long version of this post includes notes taken while i was reading Chapter 9- Working with Objects of book Java Persistence with Hibernate (http://www.manning.com/bauer2/). However, i could not post it because of the copyright rules..

 But at least, i can recommend to buy the book and read the whole chapter..

free data models

I am trying to model a complex questionnaire and during my googling for best practices i discovered this website: http://www.databaseanswers.org/data_models/
An excellent site full of free and well prepared data models on several topics..

free manual search engine

Discovering beneficial manuals, especially when newbie to a topic, technology etc.,  is vital.
 http://the-manuals.com/ seems to retrieve good documentation..

LINQ error: Null value for non-nullable member.

When i tried to save an object with a complex type (with a null instance of the complex type), i got the following error:

{"Null value for non-nullable member. Member: 'Sentence'."}

In this link there is an explanation for this: http://social.msdn.microsoft.com/Forums/en-US/adonetefx/thread/50d33271-c8e3-468f-82e1-7c3178cb4322/
According to the explanation, complex types are always considered as required in entity framework. My workaround is creating an instance of the complex type with new keyword in the constructor.

error during migration (update-database, codefirst)

I got the following error when i run Update-Database command in Package Manager Console of Visual Studio:


PM> update-database
System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.ArgumentException: The parameter is incorrect. (Exception from HRESULT: 0x80070057 (E_INVALIDARG))
   --- End of inner exception stack trace ---
   at System.RuntimeType.InvokeDispMethod(String name, BindingFlags invokeAttr, Object target, Object[] args, Boolean[] byrefModifiers, Int32 culture, String[] namedParameters)
   at System.RuntimeType.InvokeMember(String name, BindingFlags bindingFlags, Binder binder, Object target, Object[] providedArgs, ParameterModifier[] modifiers, CultureInfo culture, String[] namedParams)
   at System.Management.Automation.ComMethod.InvokeMethod(PSMethod method, Object[] arguments)
Exception has been thrown by the target of an invocation.


The solution is specifying the startup project (http://stackoverflow.com/questions/9174116/entity-framework-4-3-migration-exception-when-update-database):


Update-Database –ProjectName "MyInnerProject" –Force -ConnectionString "Data Source=.;Initial Catalog=initialcatalogname;Persist Security Info=True;User ID=userid;Password=password" -ConnectionProviderName "System.Data.SqlClient" -verbose -StartupProjectName "MyStartupProject"