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Inserting Disconnected Related Entities with Entity Framework 6

I have some data from an XML file that I have transposed and I can map to my Entities. So now I want to save them all to the database .
I was reading on SO Can Entity Framework add many related entities with single SaveChanges()?
The accepted answer does not have too much info but this statement:
'You don`t need to save changes every time if you use objects
references to newly created objects not IDs:'
My entities are derived from a dataset tables which all have a related Id columns.
And I guessing the answer is more or less that the related entity was created with something like this : item.SubItem = new SubItem();
rather than item.SubId = SubItem.Id;
So should I traverse my data tables and translate my dataset tables into Entities by creating the objects from the tables and adding them to the context.
So for each row in dt['Items'] if row has a subitem new SubItem {all values except the Id}..??
Any example code would be appreciated.

So after working to resolve the issue I discovered if I create a new object of my entity and map values to it excluding the ID column I can then assign this object back to the parent.
pseudo code is below:
myobject = new myobject(){ param1 = oldobject.param1}
myentityParent.ChildTableEntity.Add(myobject);

How and when does Entity Framework Code First generate the DB?

I came across this related question: How can I generate DDL scripts from Entity Framework 4.3 Code-First Model?
But this doesn't appear to answer the question of when a Code First application actually checks the existence/correctness of the DB and modifies it if necessary. Is it at run-time or build time? Assuming it's at run-time is it at start-up or when you create the DbContext or at the last possible moment e.g. when you try to write/read the DB table(s) it checks they exist on a case-by-case basis?

It is ceated at rutime the first time you access an entity, ie,
using (var db = new MyDBContext())
{
var items = db.MyObj.Count() // <- Here it is created!
}
There are some flavors on how, like if you set the creating strategy to CreateDatabaseIfNotExists, DropCreateDatabaseAlways, Etc. Please give this a look:
http://www.entityframeworktutorial.net/code-first/database-initialization-strategy-in-code-first.aspx

The column Model in the table __MigrationHistory is serialized and gzipped(base64) version of your EDMX. In code first the column Model is generated by Add-Migration and stored in the second part of the migration partial class and in the database when the database is created as binary stream varbinary(max).
When the database initializer (Database.SetInitializer) is called, then EF generate from the classes on the fly(Runtime) the current Entity Data Model(EDMX). The generated model will be serialized, zipped(base64) and finally compare it with the stored Model of the migration history table.
The comparison happens before the DbContext is created and, if the two Models(binary streams) are not identical then you will get a compatibility exception.

Entity Framework updating two databases

As I've mentioned in a couple other questions, I'm currently trying to replace a home-grown ORM with the Entity Framework, now that our database can support it.
Currently, we have certain objects set up such that they are mapped to a table in our internal database and a table in the database that runs our website (which is not even in the same state, let alone on the same server). So, for example:
Part p = new Part(12345);
p.Name = "Renamed part";
p.Update();
will update both the internal and the web databases simultaneously to reflect that the part with ID 12345 is now named "Renamed part". This logic only needs to go one direction (internal -> web) for the time being. We access the web database through a LINQ-to-SQL DBML and its objects.
I think my question has two parts, although it's possible I'm not asking the right question in the first place.
Is there any kind of "OnUpdate()" event/method that I can use to trigger validation of "Should this be pushed to the web?" and then do the pushing? If there isn't anything by default, is there any other way I can insert logic between .SaveChanges() and when it hits the database?
Is there any way that I can specify for each object which DBML object it maps to, and for each EF auto-generated property which property on the L2S object to map to? The names often match up, but not always so I can't rely on that. Alternatively, can I modify the L2S objects in a generic way so that they can populate themselves from the EF object?

Sounds like a job for Sql Server replication.

You don't need to inter-connect the two together as it seems you're saying with question 2.
Just have the two separate databases with their own EF or L2S models and abstract them away using repositories with domain objects.

This is the solution I ended up going with. Note that the implementation of IAdvantageWebTable is inherited from the existing base class, so nothing special needed to be done for EF-based classes, once the T4 template was modified to inherit correctly.
public partial class EntityContext
{
public override int SaveChanges(System.Data.Objects.SaveOptions options)
{
var modified = this.ObjectStateManager.GetObjectStateEntries(EntityState.Modified | EntityState.Added); // Get the list of things to update
var result = base.SaveChanges(options); // Call the base SaveChanges, which clears that list.
using (var context = new WebDataContext()) // This is the second database context.
{
foreach (var obj in modified)
{
var table = obj.Entity as IAdvantageWebTable;
if (table != null)
{
table.UpdateWeb(context); // This is IAdvantageWebTable.UpdateWeb(), which calls all the existing logic I've had in place for years.
}
}
context.SubmitChanges();
}
return result;
}
}

Same model in more than one table

I have a class, suppose it's called EntityModel, and I want to make three different tables with the same columns, as defined in EntityModel. Let's call the tables tbPast, tbPresent and tbFuture. I want also to access them separetely in the Entity DbContext:
using (var db = new MyContext())
{
var element = db.Past.Find(id);
db.Past.Remove(element);
db.Present.Add(element);
db.SaveChanges();
}
The main purpose of having three tables is performance: the table will have millions of rows, and the most important is the Present, with dozens of rows. Most queries will be made in the Present table.
What is the best way to do this? Implementing three models with the same properties doesn't seem right for me.
I'm using Entity Framework, with the Code First approach, along with ASP.NET MVC 3.

You can't use the same model to generate separate tables w/ EF code-first. If you need to have some sort of grouping, use a Discriminator field and assing it any of the values: Past Present Future.
Edit:
Similar effect can be achieved through table-per-concrete type inheritance. Thus each type will have it's own table and can share most (if not all) of the fields.

default vs custom using entity framework

I've got a table of default templates. It's global to all users. If a user has no custom template, I want to pull the default. If a user decides to customize the template it should be saved in a customtemplates table - as opposed to the globaltempaltes table.
the custom table has all the globaltemplates fields plus a userid and an id relating to which global it is replacing.
To flesh this out a bit more, lets say there are 3 templates, and a user wants to customize template 2 only. I would normaly pull the whole globaltemplates table and whatever relates to the user in the customtemplates table. Then, in the class property I'd do something in the get like this:
MyTemplateA
get { return customtemplates.A ?? globaltemplates.A; }
Can I do this using straight ef4/linq without poco?
Would a partial class with some additional properties like the get above work?
Since i'm always editing only the customtemplates table (add/edit/delete) it doesn't matter which version of the template I pull. I guess it could get hairy figuring out if it's an insert or an update.

In my opinion it will not work as you expect because EF closely relates entity to table. You cannot have single entity mapped to two tables except very special situations like splitting or inheritance.
So if you have Template entity it can be mapped only to single table but you have two. What you can do is to use TPC inheritance where Template will be a base entity mapped to GlobalTemplates table and UserTemplate will be derived entity mapped to UserTemplates table. TPC (table per concrete type) is type of inheritance where table for derived entity contains all columns from table for parent entity.
But inheritance still has a few problems for your scenario:
Template is editable - if you want to have it read only you must correctly handle it in your application logic. Any changes to attached Template instance will be saved when you call SaveChanges on the context.
When you load Template you cannot directly convert it to UserTemplate to make it user specific. You must create new instance of UserTemplate and copy properties from Template to the newly created instance.