I have used Entity Framework Code First once and although it is easy to deal with i feel like it forces you to fight your OOP principles as i tend to break many habits and design decisions just so Code First can understand my entities and map/read them from the db like:
You can't use ReadOnlyCollections
You can't have a collection of a Complex Type (Value Type)
Forced to use a hack to make enumerations work, (most market customers still have Windows XP)
and I can name a few more. What i would like to know if NHibernate supports the stuff mentioned above on Windows XP plus other things (like if it can work with SQL CE) and things that doesn't force you to change your design just to make it work.
I would like to hear an NHibernate professional/expert on this ?
Not sure about ReadOnlyCollections in particular, as NHibernate requires using interfaces and then uses its own collection implementation (which you can replace). But you can always map a private field and use a projection.
The others work out of the box.
Related
I am currently working on a large project that uses Entity Framework extensively. Part of the functionality we have implemented is dynamic querying (Filter/Sort) of the various data models based on user-supplied filters.
To achieve this I ended up using System.Linq.Dynamic which allows me, through various means, to create string-based filters like like "SomeProperty.StartsWith(#P0)" and so on, and then pass these strings (and attendant parameters) to the Dynamic Linq extension methods for IQueryable<T> (Where, etc) so that they get executed against the database and everyone is happy.
I didn't know any other way to do this at the time except for a vague
notion of Expression Trees and to be honest, I just could not get
my head around them - I spent several weeks poring over a decompilation of a component that used expressions to implement dynamic querying and I balked :)
Plus it felt like I was reinventing the wheel when the functionality I needed effectively was already written by far cleverer people than myself, in the System.Linq.Dynamic extensions.
Now the current code all works quite well as a generalised solution for filtering, sorting, etc, on any of my entities, and I'm happy enough with it however as I became more and more familiar with E.F. I started to come across things like
ObjectQuery
EntityClient
EntitySQL
Expression Trees
And I started to wonder, given that System.Linq.Dynamic is nearly 6 years old, and hasn't really had anything done with it in that time, am I missing out on anything? or, have I missed some fundamental point?
Should I bite the bullet and move my codebase over to use EntitySQL? (I assume this is like the spiritual successor to System.Linq.Dynamic, or am I wrong?)
Or should I go back and learn how to use Expression Trees because they are the way of the future/all the cool kids do it, etc? I'm not a fan of change for changes sake, and I like code that works, but I am worried that at some point in the future string-based dynamic linq becomes a dead-end and I'm stuck using it.
If anyone can help to clarify the differences between System.Linq.Dynamic and EntitySQL, or can identify any good reason for moving to Expression Trees I'd really appreciate it.
We are using Dynamic Linq extensively in our project...
Its clean and it works well, but its very complicated if you would want to peep into or change anything its code.
One of the problems what I found using a combination of Dynamic Linq and EF 6 is
EF 6 uses query caching to perform faster retrieval of data and the way where query that is built in Dynamic Linq does not use this feature of EF 6. So we have to change the where to use query caching.
This is just a small example to say Dynamic Linq is not meant for newer EF versions.
Dynamic Linq is a wonderful solution if you want to work with un-typed collection like IQuerable, but its very difficult to maintain.
I am hoping you would work in a typed environment(IQueryable). Otherwise essentially you would need to modify Dynamic Linq to really take advantage of EF 6.
Recently I've been working on some Perl projects and I'm a very novice Perl programmer. I've been experimenting with DBIx::Class and so far I'm really please with the flexibility and the ease of use. I'm curious though. I come from a .NET background and it seems like we spend a lot of time abstracting our DAL to a certain degree. Is this a good idea with a language like Perl?
Where I want to get shortly is to have the ability to start mocking my DAL so I can write unit tests for tasks. Right now though I'm struggling with how the overall structure and design of the application should look though?
Re: Relationship of the ORM within the application...
Hopefully this is the kind of answer you are looking for...
With most web app frameworks in the "scripting" world (i.e. perl, ruby, python, php), most of the time I've seen the business logic implemented at the ORM object level. E.g. in a Rails app it's at the ActiveRecord level; if you are using DBix::Class it would be at the Result-class level.
More concretely, in the case of DBIx::Class, if you have a table named VENDOR there would be a class called MySchema::Result::Vendor which represents a single row in the table VENDOR. Simply add your business methods to this class.
One disadvantage of this approach is that it ties your business logic with the ORM class which can make (unit) testing more difficult. One solution to this is to use a light-weight database for unit tests (i.e. SQLite), and an ORM like DBIx::Class will facilitate switching between the two. Of course, this won't work if you rely on SQL features which are not implemented in SQLite.
Another approach is to place your business logic methods into a Moose role. Then those methods can be composed into either the DBIx::Class Result class or into a mock object for testing. I can elaborate with an example if you'd like.
One big assumption of the above is that your business object = one row in the database. If this is not the case (i.e. you business object spans more than one table), then you'll probably want to create a "shell" or container object which has as instance members each of the constituent ORM objects. Fortunately, Moose has a nice facility for delegating methods (search for Moose delegation and the handles attribute of instance member declarations), so it is relatively easy to make a composite business object out of two or more ORM objects. Again, I can give you an example of this if you'd like.
HTH
I used to work in perl projects for the web long ago. But after working with things such as Django, perl's tools like DBI, etc now look to me rather rudimentary and outdated. Have a look at the django ORM for example, it's elegant and very productive to use, you can bypass it if your query is too complex or the ORM gets in the way...
These days I'd go python or ruby for that kind of projects.
For one liners, small text parsing or sysadmin stuff I still love to use small perl snippets. But I'm more into DRY than TMTOWTDI for more than a few lines of code these days.
Let's say I have an app that should ideally be able to use a relational database, object database, XML files, or whatever to persist its data. In the spirit of coding to interfaces instead of implementations, I have a generic DataStore interface that specifies a contract for all I/O involving the data store. This interface can be implemented by concrete classes such as RDBMSDataStore, OODBMSDataStore, XMLFileDataStore, and so on.
This works well as long as I keep the contents of the DataStore interface simple - i.e. getThis(), getThose(), saveThat(), updateThis(), etc. But as soon as I require more complicated queries, it breaks down. The XMLFileDataStore class obviously doesn't understand SQL, and the RDBMSDataStore class obviously doesn't understand XPath/XQuery. And OODBMSDataStore understands something entirely different depending on the OODBMS in use.
I could adopt a language-independent object query language, write all my queries in that and then have the concrete classes translate them into their native language, but that's a huge task, if I want to be complete.
Are there standards or best practices for handling this kind of situation in Java? Unfortunately it seems like 99% of the world interprets "database independence" to mean "relational database independence" and ignores the object databases, XML databases, document databases, etc. entirely.
From the way I read the question, this sounds a lot like the semantic that Hibernate brings to the table for Java. It even has mode for dealing with XML as the content backing store (using Dom4J). The Hibernate API has a number of extension points that could allow the addition of an OODBMS model. Even if Hibernate turns out not to be the best solution for you (implementation-wise), I think it provides a good example of the types of patterns that can be used to solve the problems you proposed.
I’m currently working on a prototype of a medium size web application, and I thought that it would be good to also experiment with Entity Framework. The problem is that the major part of the application is not the data layer and logic, and so that I don't have much time to play with Entity Framework. On the other hand, the database schema is quite simple.
One of the problems I’m facing is that I cannot find a consistent way to "write queries". As far as I can tell, there are four "interfaces" for the job:
LINQ to Entities
LINQ to Entities using LINQ extension methods
Entity SQL
Query builder
OK, the first two are essentially the same, but it’s good to use just one for maintenance and consistency.
I’m mostly puzzled by the fact that none of them seems to be complete and the most general. I often find myself cornered and using some ugly looking combination of several of them. My guess is that Entity SQL is the most general one, but writing queries using strings feels like a step back. The main reason I’m experimenting with something like Entity Framework is that I like the compile time checking.
Some other random thought / issues:
I often also use the ObjectQuery.Include() method, but again it takes a string. Is this the only way?
When to use ObjectQuery.Execute() (vs. ToList())? Does it actually execute the query?
Should execute queries as soon as possible (e.g. using ToList()) or should I not care just let leave the execution for the first enumeration which gets in the way?
Are ObjectQuery.Skip() and ObjectQuery.Take() available only as extension methods? Is there a better way to do paging? It’s 2009 and almost every web application deals with paging.
Overall, I understand there are many difficulties when implementing an ORM, and often one has to compromise. On the other hand, the direct database access (e.g. ADO.NET) is plain and simple and has well defined interface (tabular results, data readers), so all code - no matter who and when writes it - is consistent. I don’t want to faced with too many choices whenever writing a database query. It’s too tedious and more than likely different developers will come up with different ways.
What are your rules of thumbs?
I use LINQ-to-Entities as much as possible. I also try and formalise to the lambda-form, as opposed to the extended SQL-style syntax. I have to admit to have had problems enforcing relationships and making compromises on efficiency just to expedite my coding of our application (eg. Master->Child tables may need to be manually loaded) but all in all, EF is a good product.
I do use EF's .Include() method for lazy-loading, which as you say, does require a string input. I find no problem with this, other than that of identifying the string to use which is relatively simple. I guess if you're keen on compile-time checking of such relations, a model similar to: Parent.GetChildren() might be more appropriate.
My application does require some "dynamic" queries to be performed, though. I have two ways of meeting this:
a) I create a mediator object, eg. ClientSearchMediator, which "knows" how to search for clients by name, etc. I can then put this through a SearchHandler.Search(ISearchMediator[] mediators) call (for example). This can be used to target specific data structures and sort results accordingly using LINQ-to-Entities.
b) For a looser experience, possibly as a result of a user designing their own query (using high level tools our application provides), eSQL is ideal for this purpose. It can be made to be injection-safe.
I don't have enough knowledge to address all of this, but I'll at least take a few stabs.
I don't know why you think ADO.NET is more consistent than Entity Framework. There are many different ways to use ADO.NET and I've definitely seen inconsistency within a single code base.
Entity Framework is currently a 1.0 release and it suffers from many 1.0 type problems (incomplete & inconsistent API, missing features, etc.).
In regards to Include, I assume you are referring to eager loading. Multiple people (outside of Microsoft) have developed solutions for getting "type safe" includes (try googling something like: Entity Framework ObjectQueryExtension Include). That said, Include is more of a hint than anything. You can't force eager loading and you have to always remember to call the IsLoaded() method to see if your request was fulfilled. As far as I know, the way "Include" works is not changing at all in the next version of Entity Framework (4.0 - to ship with VS 2010).
As far as executing the Linq query as soon as it's built vs. the last possible moment, that decision is situational. Personally, I would probably execute it as soon as it's built for the most part unless there was a compelling reason not to, but I can see other people going the opposite direction.
There are more mature ORMs on the market and Entity Framework isn't necessarily your best option. For the most part, you can bend Entity Framework to your will, but you may end up rolling your own implementation of features that come out of the box with other ORMs.
Following the way Rob does it, I have the classes that are generated by the Linq to SQL wizard, and then a copy of those classes that are POCOs. In my repositories I return these POCOs rather than the Linq to SQL models:
return from c in DataContext.Customer
where c.ID == id
select new MyPocoModels.Customer { ID = c.ID, Name = c.Name }
I understand that the benefit of this is that the POCO models can be instantiated easier so this will make my code more testable.
I'm now moving from Linq to SQL over to Entity Framework and I'm about half way through an EF book. It seems there's a lot of goodness I'm going to lose out on by returning POCOs from my repositories rather than the EF entities.
I still haven't really embraced unit testing, so I feel like I'm wasting a lot of time creating these extra POCOs and writing the code to populate them, when all I appear to be gaining is testable code, yet I'm also gonna lose out on a lot of the benefits of the EF by not being able to track my objects.
Does anyone have any advice for a relative newb to all this ORM/Repository stuff?
Anthony
Another reason people don't like the auto-generated objects (in LINQ to SQL for example) is because of their built-in "magic".
Usually the magic is invisible and you never notice it, but when you try to do things like serialize one of those objects and then deserialize it (for example when using web services) its internal connection to the data source is broken and special hacks need to be employed to "put the magic back in".
With POCOs, you don't have to worry about those sorts of things and can get a better separation between your data and service layers. The downside of course is that you have to write lots of boring POCO -> magic object and magic object -> POCO conversion code. But in the end I think it's usually worth it, especially for large or complex projects.
The main reason is that a lot of people like to develop their model with a specific mindset: like DDD for instance. They might want to use a specific pattern (like Spec or State) for things like statuses (instead of enums) - or you might want to use a Factory for instantiation.
OO breaks when you try to use Tables as Objects when things get more complex. Simple sites work OK - but when you get to big big things, it gets ugly.
So - as always - it depends what you think your project will turn into.
My experience is that when you start writting some complex queries .Include method is worthless and you will find yourself either:
a) Writting a lot of queries to get the data you want or
b) abusing of annonymous types to load the data in a single query and then writting a lot of code just to pass that data to your entities.
POCOs are the way to go, IMHO.