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Adding functionality to my Entity Framework POCO classes

I've built a web application with Entity Framework using POCO.
I'm using these POCO classes as my business objects and not just for persisting data which works fine until...
Now I need to add some logic into these classes to do thing like total up sales, order lines, etc.
Should I add methods to my POCO classes to enable this functionality or leave them purely for persisting data and create some kind of 'processor' whereby I pass in the business objects and get the values I require out.
Is there a best practice for this?

What is the architectural design you are using or want to use?
For example, if these are your domain entities, you should put as much as possible logic in them. If they are merely data containers and you don't have a real architecture in place, your logic would probably in some business component.
So if you provide your question with some more details, we can help you better.

Why EF Entities are making a dependency and the need of POCO

I'm pretty new to IoC, Dependency Injection and Unit Testing. I'm starting a new pet project and I'm trying to do it right.
I was planning to use the Repository pattern to mediate with the data. The objects that I was going to return from the repositories were going to be objects collected from a Linq to entities data context (EF4).
I'm reading in "Dependency Injection" from Mark Seeman that doing it, makes an important dependency and will definitely complicate the testing (that's what he's using POCO objects in a Library Project).
I'm not understanding why. Although the objects are created by a linq to entities context, I can create them simply calling the constructor as they were normal objects. So I assume that is possible to create fake repositories that deviler these objects to the caller.
I'm also concerned about the automatic generation of POCO classes, which is not very easy.
Can somebody bring some light? Are POCO objects trully necessary for a decoupled and testable project?
**EDIT: Thanks to Yuck I understand that it's better to avoid autogeneration with templates, which brings me to a design question. If I come from a big legacy database wich his tables are assuming a variety of responsabilities (doesn't fit well with the concept of a class with a single responsability), what's the best way to deal with that?
Delete the database is not an option ;-)

No, they're not necessary it just makes things easier, cleaner.
The POCO library won't have any knowledge that it's being used by Entity Framework. This allows it to be used in other ways - in place of a view model, for instance. It also allows you to use the same project on both sides of a WCF service which eliminates the need to create data transfer objects (DTO).
Just two examples from personal experience but there are surely more. In general the less a particular object or piece of code knows about who is using it or how it's being used will make it more adaptable and generic for other situations.
You also mention automatic generation of POCO classes. I don't recommend doing this. Were you planning to generate the class definitions from your database structure?

I was planning to use the Repository pattern to mediate with the data.
The objects that I was going to return from the repositories were
going to be objects collected from a Linq to entities data context
(EF4).
The default classes (not the POCOs) EF generates contain proxies for lazy loading and are tied at the hip to Entity Framework. That means any other class that wants to use those classes will have to reference the required EF assemblies.
This is the dependency Mark Seeman is talking about. Since you are now dependent on these non-abstract types, which in turn are dependent on EF, you cannot simply change the implementation of your repository to something different (i.e. just using your own persistence store) without addressing this change in the class that depend on these types.
If you are truly only interested in the public properties of the EF generated types then you can have the partial classes generated by EF implement a base interface. Put all the properties you need in that base interface and pass the dependency in as the base interface - now you only depend on the base interface and not EF anymore.

Rich domain model with ORM

I seem to be missing something and extensive use of google didn't help to improve my understanding...
Here is my problem:
I like to create my domain model in a persistence ignorant manner, for example:
I don't want to add virtual if I don't need it otherwise.
I don't like to add a default constructor, because I like my objects to always be fully constructed. Furthermore, the need for a default constructor is problematic in the context of dependency injection.
I don't want to use overly complicated mappings, because my domain model uses interfaces or other constructs not readily supported by the ORM.
One solution to this would be to have separate domain objects and data entities. Retrieval of the constructed domain objects could easily be solved using the repository pattern and building the domain object from the data entity returned by the ORM. Using AutoMapper, this would be trivial and not too much code overhead.
But I have one big problem with this approach: It seems that I can't really support lazy loading without writing code for it myself. Additionally, I would have quite a lot of classes for the same "thing", especially in the extended context of WCF and UI:
Data entity (mapped to the ORM)
Domain model
WCF DTO
View model
So, my question is: What am I missing? How is this problem generally solved?
UPDATE:
The answers so far suggest what I already feared: It looks like I have two options:
Make compromises on the domain model to match the prerequisites of the ORM and thus have a domain model the ORM leaks into
Create a lot of additional code
UPDATE:
In addition to the accepted answer, please see my answer for concrete information on how I solved those problems for me.

I would question that matching the prereqs of an ORM is necessarily "making compromises". However, some of these are fair points from the standpoint of a highly SOLID, loosely-coupled architecture.
An ORM framework exists for one sole reason; to take a domain model implemented by you, and persist it into a similar DB structure, without you having to implement a large number of bug-prone, near-impossible-to-unit-test SQL strings or stored procedures. They also easily implement concepts like lazy-loading; hydrating an object at the last minute before that object is needed, instead of building a large object graph yourself.
If you want stored procs, or have them and need to use them (whether you want to or not), most ORMs are not the right tool for the job. If you have a very complex domain structure such that the ORM cannot map the relationship between a field and its data source, I would seriously question why you are using that domain and that data source. And if you want 100% POCO objects, with no knowledge of the persistence mechanism behind, then you will likely end up doing an end run around most of the power of an ORM, because if the domain doesn't have virtual members or child collections that can be replaced with proxies, then you are forced to eager-load the entire object graph (which may well be impossible if you have a massive interlinked object graph).
While ORMs do require some knowledge in the domain of the persistence mechanism in terms of domain design, an ORM still results in much more SOLID designs, IMO. Without an ORM, these are your options:
Roll your own Repository that contains a method to produce and persist every type of "top-level" object in your domain (a "God Object" anti-pattern)
Create DAOs that each work on a different object type. These types require you to hard-code the get and set between ADO DataReaders and your objects; in the average case a mapping greatly simplifies the process. The DAOs also have to know about each other; to persist an Invoice you need the DAO for the Invoice, which needs a DAO for the InvoiceLine, Customer and GeneralLedger objects as well. And, there must be a common, abstracted transaction control mechanism built into all of this.
Set up an ActiveRecord pattern where objects persist themselves (and put even more knowledge about the persistence mechanism into your domain)
Overall, the second option is the most SOLID, but more often than not it turns into a beast-and-two-thirds to maintain, especially when dealing with a domain containing backreferences and circular references. For instance, for fast retrieval and/or traversal, an InvoiceLineDetail record (perhaps containing shipping notes or tax information) might refer directly to the Invoice as well as the InvoiceLine to which it belongs. That creates a 3-node circular reference that requires either an O(n^2) algorithm to detect that the object has been handled already, or hard-coded logic concerning a "cascade" behavior for the backreference. I've had to implement "graph walkers" before; trust me, you DO NOT WANT to do this if there is ANY other way of doing the job.
So, in conclusion, my opinion is that ORMs are the least of all evils given a sufficiently complex domain. They encapsulate much of what is not SOLID about persistence mechanisms, and reduce knowledge of the domain about its persistence to very high-level implementation details that break down to simple rules ("all domain objects must have all their public members marked virtual").

In short - it is not solved
(here goes additional useless characters to post my awesome answer)

All good points.
I don't have an answer (but the comment got too long when I decided to add something about stored procs) except to say my philosophy seems to be identical to yours and I code or code generate.
Things like partial classes make this a lot easier than it used to be in the early .NET days. But ORMs (as a distinct "thing" as opposed to something that just gets done in getting to and from the database) still require a LOT of compromises and they are, frankly, too leaky of an abstraction for me. And I'm not big on having a lot of dupe classes because my designs tend to have a very long life and change a lot over the years (decades, even).
As far as the database side, stored procs are a necessity in my view. I know that ORMs support them, but the tendency is not to do so by most ORM users and that is a huge negative for me - because they talk about a best practice and then they couple to a table-based design even if it is created from a code-first model. Seems to me they should look at an object datastore if they don't want to use a relational database in a way which utilizes its strengths. I believe in Code AND Database first - i.e. model the database and the object model simultaneously back and forth and then work inwards from both ends. I'm going to lay it out right here:
If you let your developers code ORM against your tables, your app is going to have problems being able to live for years. Tables need to change. More and more people are going to want to knock up against those entities, and now they all are using an ORM generated from tables. And you are going to want to refactor your tables over time. In addition, only stored procedures are going to give you any kind of usable role-based manageability without dealing with every tabl on a per-column GRANT basis - which is super-painful. If you program well in OO, you have to understand the benefits of controlled coupling. That's all stored procedures are - USE THEM so your database has a well-defined interface. Or don't use a relational database if you just want a "dumb" datastore.

Have you looked at the Entity Framework 4.1 Code First? IIRC, the domain objects are pure POCOs.

this what we did on our latest project, and it worked out pretty well
use EF 4.1 with virtual keywords for our business objects and have our own custom implementation of T4 template. Wrapping the ObjectContext behind an interface for repository style dataaccess.
using automapper to convert between Bo To DTO
using autoMapper to convert between ViewModel and DTO.
you would think that viewmodel and Dto and Business objects are same thing, and they might look same, but they have a very clear seperation in terms of concerns.
View Models are more about UI screen, DTO is more about the task you are accomplishing, and Business objects primarily concerned about the domain
There are some comprimises along the way, but if you want EF, then the benfits outweigh things that you give up

Over a year later, I have solved these problems for me now.
Using NHibernate, I am able to map fairly complex Domain Models to reasonable database designs that wouldn't make a DBA cringe.
Sometimes it is needed to create a new implementation of the IUserType interface so that NHibernate can correctly persist a custom type. Thanks to NHibernates extensible nature, that is no big deal.
I found no way to avoid adding virtual to my properties without loosing lazy loading. I still don't particularly like it, especially because of all the warnings from Code Analysis about virtual properties without derived classes overriding them, but out of pragmatism, I can now live with it.
For the default constructor I also found a solution I can live with. I add the constructors I need as public constructors and I add an obsolete protected constructor for NHibernate to use:
[Obsolete("This constructor exists because of NHibernate. Do not use.")]
protected DataExportForeignKey()
{
}

Best pattern to implement Translators between DB type object to Business type object

Our project had implemented Entity framework and we were floating the entity object type across all layers. Due to the sheer size of those objects we have decided to have business entities and data entities. Now I am confused as in
1) Where to implement the translation of these objects
2) How i.e any patterns or practices which I should follow to get better results.
*Edited *
Thanks for all the replies I am looking into Automapper and found it might fit well. But instead of using a library I wanted to create my own translator which would seem like reinventing the wheel. But my thought is I would have a lot more control on things then.
So going back to the original quesiton
On which layer should I implement my translation i.e we have an N-tire app and there are two schools of thought one being we should have a business layer and treat it as a facade and implement all translation and other business logic there and treat all others layer as a dumb layer which have no knowledge about the other layers and keep minimum reference of them so the data layer would be throwing out the same data entities and can be reused in another project where the Business object are somewhat different then our current object. The second school of thought is that the data layer should be returning and accepting business entities which would restrict non-database developers from unnecessary call on the db layer to create a business entitie.
Any patterns which are there for such kind of translation libraries.

This sounds similar to the problem faced by many people when using the MVVM pattern in association with an ORM. Automapper (http://automapper.codeplex.com/) is a great library that provides clean, convention based translation between types.

Please take a look the following, it has a really nice overview of all of microsoft techs. It might be a bit complicated though but it shows the layers very nicely.
N Layer sample app

You can try DTO's and ValueInjecter if you are moving data over the wire, or the same tool if you use ViewModels to bind to Views ... AutoMapper is great too.

.net, C# Interface between Business Logic and DAL

I'm working on a small application from scratch and using it to try to teach myself architecture and design concepts. It's a .NET 3.5, WPF application, and I'm using Sql Compact Edition as my data store.
I'm working on the business logic layer, and have just now begun to write the DAL. I'm just using SqlCeComamnds to send over simple queries and SqlCeResultSet to get at the results. I'm starting to design my Insert and Update methods, and here's the issue - I don't know the best way to get the necessary data from the BLL into the DAL. Do I pass in a generic collection? Do I have a massive parameter list with all the data for the database? Do I simply pass in the actual business object (thus tying my DAL to the conrete stuff in the BLL?).
I thought about using interfaces - simply passing IBusinessObjectA into the DAL, which provides the simplicity I'm looking for without tying me TOO tightly to current implementations. What do you guys think?

I don't think there is a simple answer to your questions because there are many options depending on the circumstances. I have found it helpful to read the two books below to help me understand the problems you describe better.
MS .NET: Architecting Applications for the Enterprise (Esposito, Saltarello)
MS Application Architecture Guide, 2nd edition.
The second book is available online. Look here.

I think it is OK to pass the Business object to the Data Access Layer. I think the BLL's job is just to work with its objects, to check if all rules are being followed, about what can be saved, by whom, on what fields, time, etc.
Once it has done that it should pass it to the DAL, and I think it is IT'S job to figure out how to convert what it got into something that can be persisted, but it wont check what is being persisted or read or by whom, it will just do it. This could be straight foward, a la linq, but if your logic mdoels do not match your data model 1:1, then the DAL should do all the conversion.
About tying your DAL to the stuff in the BLL, I think you should worry about the other way around, tying your BLL to your DAL. I would use an interface to represent your DAL (as in IRepository) that way you can make your BLL call any kind of persistance mechanism just by changing the type of IRepository it is using (extra points if you use IoC :P). The concrete classes that implement the IRepository would be tied to the business objects, but they have to know what is it that they are saving don't they? while the BLL does NOT have to know what is doing the saving.

To pass business object in the DAL is the simpler and fastest method. It works in small projects, but have same disadvantages:
1) Business Objects are part of BLL layer, and if you pass objects in BLL then DAL becomes dependent of BLL. low layer knows about upper one - this contradicts the idea of layers at all.
2) Business Object are usially very complex to save it directly in BD. In this case it is better to introduce new "Mappers" intermediate layer.
To overcome all these issues I usially make interface to DAL independent of Business Objects. I use "Row" classes instead - representation of one record in the database or XML. In .NET 3.5 linqtosql autogenerated classes can be used for this purpose.

If I was in your position, I'd probably use LINQ to SQL to define my data access layer - it'll save you lots of work maintaining all that SqlCeFooBar stuff and give you a designer (of sorts) for maintaining your database that you would otherwise lack, using SQL CE.
So in that case, I'd probably couple the business logic layer pretty tightly to the entities exposed by the L2S layer. The justification being that the entities are the business objects, albeit devoid of any services.
I probably wouldn't let the entities get as far up the hierarchy as the UI though. At that level, it makes much more sense to use a model specifically for the view - especially given that you're using WPF.
Of course, all of this depends upon the size and complexity of your application. I'm assuming it's a fairly small scale application (single user?) given that you're using SQL CE.