I have an MVC3 NHibernate/ActiveRecord project. The project is going okay, and I'm getting a bit of use out of my model objects (mostly one giant hierarchy of three or four classes).
My application is analytics based; I store hierarchial data, and later slice it up, display it in graphs, etc. so the actual relationship is not that complicated.
So far, I haven't benefited much from ORM; it makes querying easy (ActiveRecord), but I frequently need less information than full objects, and I need to write "hard" queries through complex and multiple selects and iterations over collections -- raw SQL would be much faster and cleaner.
So I'm thinking about ditching ORM in this case, and going back to raw SQL. But I'm not sure how to rearchitect my solution. How should I handle the database tier?
Should I still have one class per model, with static methods to query for objects? Or should I have one class representing the DB?
Should I write my own layer under ActiveRecord (or my own ActiveRecord-like implementation) to keep the existing code more or less sound?
Should I combine ORM methods (like Save/Delete) into my model classes or not?
Should I change my table structure (one table per class with all of the fields)?
Any advice would be appreciated. I'm trying to figure out the best architecture and design to go with.
Many, including myself, think the ActiveRecord pattern is an anti-pattern mainly because it breaks the SRP and doesn't allow POCO objects (tightly coupling your domain to a particular ORM).
In saying that, you can't beat an ORM for simple CRUD stuff, so I would keep some kind of ORM around for that kind of work. Just re-architect your application to use POCO objects and some kind or repository pattern with your ORM implementation specifics in another project.
As for your "hard" queries, I would consider creating one class per view using a tiny ORM (like Dapper, PetaPoco, or Massive), to query the objects with your own raw sql.
Related
What I see a lot is that people use a Object Relational Mapper (ORM) for doing SQL stuff when working in a MVC environment. But if i really have complex queries I would like to write this whole query myself. What is the best practice for this kind of situation?
Having a Abstraction Layer between your model and the database with the complex queries
Still using the model with creating specific methodes that handle the queries
Or is there any other way that might be better? please tell me :)
Consider the Single Responsibility Principle. Specifically, the question would be...
"If I put data access logic in my model, what will that mean when I need to change something?"
Any time you need to change business logic, you're also changing the objects which maintain data access logic. So the data access logic also needs to be re-tested. Conversely, any time you need to change data access logic, you're also changing the objects which maintain business logic. So the business logic also needs to be re-tested.
As the logic expands, this becomes more difficult very quickly.
The idea behind the Single Responsibility Principle is to separate the dependencies of different roles which can enact changes to the application. (Keep in mind that "roles" doesn't map 1-to-1 with "people." One person may have multiple roles, but it's still important to separate those roles.) It's a matter of simpler support. If you want to make a change to a database query (say, for performance reasons) which shouldn't have any visible affect on anything else in the system, then there's no reason to be changing objects which contain business logic.
1. Having a Abstraction Layer between your model and the database with the complex queries
Yes, you should have a persistence abstraction that sits between storage (database or any other data source) and you business logic. Your business logic should not depend on "where", "how" and even "if" the data is actually stored.
Basically, your code should (at least - try to) adhere to SOLID principles, but as #david already pointed out: you are already violating the first on on that list.
Also, you should consider using a service layer which would be responsible for dealing with interaction between implementation of domain model and your persistence abstraction (doesn't matter whether you are using custom written data mappers or some 3rd party ORM).
In the article (more like excerpt, actually) the "MVC model" is actually all three concentric circles together. Domain model is not code. It actually is trm that describs the accumulated knowledge about the project. Most of domain model gets turned into pieces of code. Those pieces are referred to as domain objects.
2. Still using the model with creating specific methodes that handle the queries
This would imply implementation of active record. It is useful, but mostly misused pattern, for cases when your objects have no (or almost none) business logic. Basically - you should use active record only if all you need are glorified setter an getters, that talk to database.
Active record pattern is a very good choice when you need to quickly prototype something, but it should not be used, when you are attempting to implement fully realized model layer.
ORM's in general do not specifically have any drawbacks versus using direct SQL to fetch data from the database. ORM's as the name implies help in keeping your Relational model (designed using your SQL DDL's or using JPA annotations) and OO model in sync and help them integrate well together.
When using a ORM, you can write your queries in JPQL which is Object oriented SQL. So instead of writing queries that manipulate tables, you are writing queries that manipulate objects. You use the relationships between these objects to get your desired result. Now I understand that sometimes its easier to just write Native SQL, so the JPA specification allows you to run native sql! This just returns you list of "Generic Objects" which you can organize any way you like. When you choose to go this route and actually pick a JPA provider, like Hibernate, these providers have extended functionalities. So if you do have complex relationships you can use libraries like Hibernate Criteria Builder to help you create queries for those complex relationships.
So, if building a large MVC application, it would generally be a good idea to have this abstraction layer in the middle - handling all these relationships. It makes it easier on you the developer to just look at the big picture and the business side of the application.
Imho, no. I think, even the ORM layer adds often more complexity as needed. The databases have very good and sophisticated mechanisms for high-level data manipulation. Triggers, views, constraints, complex keying-indexing, (sub)transactions, stored procedures, and procedural extensions of the query language were normally much more as enough for everything.
The ORMs can't give, because of their structural barriers, a real interface to this feature set.
And the common practice is that the applications use practically only a nosql record service from all of this, and implement in an unneeded "middleware" which were the mission of the database.
Which I see really interesting, if the feature set of the databases got some OO-like interface (see "sql abstract types"), and the client-side logic went in the application (see "REST"). This practically eliminated the need of the middle layer.
When using LINQ to SQL or Entity framework,shall we need to separate application in 3 layers?BLL,DAL,Interface?
Do what works for you. Building a wedding website with a handful of links and getting 5 content pages out of the database? More than 1 layer seems like tremendous overkill. On the flip side, for a very complex or large project: I think you'd want at least some degree separation because it saves time, confusion and sanity.
It matters what you're working on and how much division it requires. Ultimately it's what you and your team prefer. There's no right answer, it's what fits the situation.
in projects I've been developing, I find value in creating a DL even when using Linq2Sql for data access.
My main reason is because many of the calls to the DL, to retreive one or more business objects from the DB, actually require more than one call to the database, especially when implementing an eager-loading strategy. and when saving a business object, whose data is stored in multiple tables, a transaction can be established across multiple calls to the database.
The business layer doesn't need to know that; it should be able to make a single call to the DL and leave it to the DL to do all the tedious querying and collation of data into business objects.
I'm with #MikeJacobs.
I've actually written a LINQ2SQL library which abstracts ALL the DataContext stuff, and all the .Insert(), .Execute() and .SubmitChanges().
It's really nice to just abstract that away. In LINQ2SQL, you're still dependant on all your layers knowing about the LINQ2SQL Entities, but my top layers is very rarely sending complex lambdas to the DAL, most of that's done in the DAL.
I am building a DataAccess layer to a DB, what data structure is recommended to use to pass and return a collection?
I use a list of data access objects mapped to the db tables.
I'm not sure what language you're using, but in general, there are tradeoffs of simplicity vs extensibility.
If you return the DataSet directly, you have now coupled yourself to database specific classes. This leaves little room for extension - what if you allow access to files or to other types of data sources? But, it is also very simple. This is the recordset pattern and C#/VB provide a lot of built-in support for this. The GUI layer can access the recordset and easily manipulate the data. This works well for simple applications.
On the other hand, you can wrap the datasets in a custom object, and provide gateway methods (see the Gateway pattern http://martinfowler.com/eaaCatalog/gateway.html). This method is more complex, but provides a lot more extensibility. In a larger application when you need to separate the the business logic, data logic, and GUI logic, this is a more robust way to go.
For larger enterprise applications, you can look into using Object Relational Mapping tools (ORM). They help to automatically map java objects to database tables. They hide a lot of the painful SQL details. Frameworks such as Spring provide excellent support for ORMs.
I tend to use arrays of objects, so that I can disconnect the DAO from the business logic.
You can store the data in the DAO as a dataset, for example, and give them an easy way to add to the database before doing an update, so they can pass in information to do modification operations, and then when they want to commit the changes they can do it in one shot.
I prefer that the user can't add/modify the structure themselves, as it makes it harder to determine what must be changed in the database.
By initially returning an array they can then display what is in the database.
Then, as the presentation layer makes changes, the DAO can be updated by the controller. By having a loose coupling the entire system becomes more flexible, as you can change the DAO from a dataset to something else, and the rest of the application doesn't care.
There are two choices that are the most generic.
The first way to look at a ResultSet is as a List of Maps, where each Map represents a row in the ResultSet. The keys are the columns listed in the FROM clause; the values are the database values.
The second way to look at a ResultSet is as a Map of Lists, where each List represents a column in the ResultSet. The Map keys are the columns listed in the FROM clause; the values are the List of database values.
If you don't want to do full-blown ORM, these can carry you a long way.
In MVC, is the ORM the same as the model or just a way the model can be designed? In other words, the "model" doesn't care how you get data as long as you get it. Or, does "model" imply that I no longer have a bunch of SQL statements in my code like in code behind forms? Something else?
Thank you.
No, the ORM is the thing that maps a code-based model to your database and vice versa.
For basic CRUD apps, where your model in code is literally just DTOs that represent the database and you're loading, editing, and saving them, that's how you'd use it. If you do have a "proper" Domain Model, then it's a bit more complex because ideally you'd want to decouple the shape of the Domain Model classes from the shape of the database tables.
To elaborate, you would create your model in your code to represent the Domain Model (i.e. the various elements of your problem domain), build some sort of "memento" classes that are pure DTOs that you can convert your Domain Model classes from/into. Then configure an ORM (object relational mapper) to map those memento DTOs to a database. I.e. Generate SQL statements that will update the database based on the model objects you give to it.
I can understand some confusion, because there are tools (LINQ to SQL being one) that actually generate model classes in a designer for you. This isn't pure ORM, like NHibernate, where you provide the ORM plain old objects and some mapping configuration that it uses (often in conjunction with reflection) to automatically generate the SQL statements for the database. You could possibly get away with using EF Code First to map a Domain Model directly to the database, but I think in the end it may get messy as you try to make changes to one or the other.
If you'd like to have a look at a good real world implementation of MVC with an ORM, have a look at S#arp Architecture which is based on MS ASP.NET MVC, Nhibernate and the repository pattern.
The model should be decoupled from the backend data store technology as much as possible.
I thought this was a pretty good article that discusses the relationship between data access layers, DTOs, etc. http://msdn.microsoft.com/en-us/magazine/dd263098.aspx
As someone who hasn't used either technology on real-world projects I wonder if anyone knows how these two complement each other and how much their functionalities overlap?
LINQ to SQL forces you to use the table-per-class pattern. The benefits of using this pattern are that it's quick and easy to implement and it takes very little effort to get your domain running based on an existing database structure. For simple applications, this is perfectly acceptable (and oftentimes even preferable), but for more complex applications devs will often suggest using a domain driven design pattern instead (which is what NHibernate facilitates).
The problem with the table-per-class pattern is that your database structure has a direct influence over your domain design. For instance, let's say you have a Customers table with the following columns to hold a customer's primary address information:
StreetAddress
City
State
Zip
Now, let's say you want to add columns for the customer's mailing address as well so you add in the following columns to the Customers table:
MailingStreetAddress
MailingCity
MailingState
MailingZip
Using LINQ to SQL, the Customer object in your domain would now have properties for each of these eight columns. But if you were following a domain driven design pattern, you would probably have created an Address class and had your Customer class hold two Address properties, one for the mailing address and one for their current address.
That's a simple example, but it demonstrates how the table-per-class pattern can lead to a somewhat smelly domain. In the end, it's up to you. Again, for simple apps that just need basic CRUD (create, read, update, delete) functionality, LINQ to SQL is ideal because of simplicity. But personally I like using NHibernate because it facilitates a cleaner domain.
Edit: #lomaxx - Yes, the example I used was simplistic and could have been optimized to work well with LINQ to SQL. I wanted to keep it as basic as possible to drive home the point. The point remains though that there are several scenarios where having your database structure determine your domain structure would be a bad idea, or at least lead to suboptimal OO design.
Two points that have been missed so far:
LINQ to SQL does not work with Oracle
or any database apart from SqlServer. However 3rd parties do offer better support for Oracle, e.g. devArt's dotConnect, DbLinq, Mindscape's LightSpeed and ALinq. (I do not have any personal experience with these)
Linq to NHibernate lets you used
Linq with a Nhiberate, so it may
remove a reason not to use.
Also the new fluent interface to Nhibernate seems to make it less painful to configure Nhibernate’s mapping. (Removing one of the pain points of Nhibernate)
Update
Linq to Nhiberate is better in Nhiberate v3 that is now in alpha. Looks like Nhiberate v3 may ship towards the end of this year.
The Entity Frame Work as of .net 4 is also starting to look like a real option.
#Kevin: I think the problem with the example you are presenting is that you are using a poor database design. I would have thought you'd create a customer table and an address table and normalized the tables. If you do that you can definately use Linq To SQL for the scenario you're suggesting. Scott Guthrie has a great series of posts on using Linq To SQL which I would strongly suggest you check out.
I don't think you could say Linq and NHibernate complement each other as that would imply that they could be used together, and whilst this is possible, you're much better off choosing one and sticking to it.
NHibernate allows you to map your database tables to your domain objects in a highly flexible way. It also allows you to use HBL to query the database.
Linq to SQL also allows you to map your domain objects to the database however it use the Linq query syntax to query the database
The main difference here is that the Linq query syntax is checked at compile time by the compiler to ensure your queries are valid.
Some things to be aware of with linq is that it's only available in .net 3.x and is only supported in VS2008. NHibernate is available in 2.0 and 3.x as well as VS2005.
Some things to be aware of with NHibernate is that it does not generate your domain objects, nor does it generate the mapping files. You need to do this manually. Linq can
do this automatically for you.
Fluent NHibernate can generate your mapping files based on simple conventions. No XML-writing and strongly typed.
I've recently worked on a project, where we needed to change from Linq To SQL to NHibernate for performance reasons. Especially L2S's way of materializing the objects seems slower than NHibernate's ditto and the change management is quite slow too. And it can be hard to turn the change management off for specific scenarios where it is not needed.
If you are going to use your entities disconnected from the DataContext - in WCF scenarios for example - you're may have a lot of trouble connecting them to the DataContext again for updating the changes. I have had no problems with that with NHibernate.
The thing I will miss from L2S is mostly the code generation that keeps relations up-to-date on both ends of the entities. But I guess there are some tools for NHibernate to do that out there too...
Can you clarify what you mean by "LINQ"?
LINQ isn't an data access technology, it's just a language feature which supports querying as a native construct. It can query any object model which supports specific interfaces (e.g. IQueryable).
Many people refer to LINQ To SQL as LINQ, but that's not at all correct. Microsoft has just released LINQ To Entities with .NET 3.5 SP1. Additionally, NHibernate has a LINQ interface, so you could use LINQ and NHibernate to get at your data.
By LINQ, I'm assuming you mean LINQ to SQL because LINQ, by itself, has no database "goings on" associated with it. It's just an query language that has a boat-load of syntac sugar to make it look SQL-ish.
In the very basic of basic examples, NHibernate and LINQ to SQL seem to both be solving the same problem. Once you get pass that you soon realize that NHibernate has support for a lot of features that allow you to create truly rich domain models. There is also a LINQ to NHibernate project that allows you to use LINQ to query NHibernate in much the same way as you would use LINQ to SQL.
First let´s separate two different things:
Database modeling is concerned about the data while object modeling is concerned about entities and relationships.
Linq-to-SQL advantage is to quickly generate classes out of database schema so that they can be used as active record objects (see active record design pattern definition).
NHibernate advantage is to allow flexibility between your object modeling and database modeling. Database can be modeled to best reflect your data taking in consideration performance for instance. While your object modeling will best reflect the elements of the business rule using an approach such as Domain-Driven-Design. (see Kevin Pang comment)
With legacy databases with poor modeling and/or naming conventions then Linq-to-SQL will reflect this unwanted structures and names to your classes. However NHibernate can hide this mess with data mappers.
In greenfield projects where databases have good naming and low complexity, Linq-to-SQL can be good choice.
However you can use Fluent NHibernate with auto-mappings for this same purpose with mapping as convention. In this case you don´t worry about any data mappers with XML or C# and let NHibernate to generate the database schema from your entities based on a convention that you can customize.
On the other hand learning curve of Linq-to-SQL is smaller then NHibernate.
Or you could use the Castle ActiveRecords project. I've been using that for a short time to ramp up some new code for a legacy project. It uses NHibernate and works on the active record pattern (surprising given its name I know). I haven't tried, but I assume that once you've used it, if you feel the need to drop to NHibernate support directly, it wouldn't be too much to do so for part or all of your project.
As you written "for a person who have not used either of the them"
LINQ to SQL is easy to use so any one can use it easily
It also support procedures, which helps most of the time.
Suppose you want to get data from more than one table then write a procedure and drag that procedure to designer and it will create everything for you,
Suppose your procedure name is "CUSTOMER_ORDER_LINEITEM" which fetch record from all these three table then just write
MyDataContext db = new MyDataContext();
List<CUSTOMER_ORDER_LINEITEMResult> records = db.CUSTOMER_ORDER_LINEITEM(pram1, param2 ...).ToList<CUSTOMER_ORDER_LINEITEMResult>();
you can use you records object in foreach loop as well, which is not supported by NHibernate