I have learn about Object role modeling but not about Object-relational mapping and I want to know if they are two ways of doing the same thing and what are the pros and cons? To me Object role modeling makes a lot more sense. Could you make a brief but easy to understand comparison if they can be compared. Cheers
Object Role Modeling: software modeling notation to, specially, define domain models. You can think of this language as an alternative to using UML class diagrams to design your database. More info here: http://www.orm.net/
Object-relational mapping: a set of strategies to bridge the gap between object-oriented programs and relational databases. It aims to allow the persistent storage of objects in a relational database structure
Object Role Modeling was invented by a team at Control Data around 1973, and named by Falkenberg. It is a modeling method rooted in linguistic analysis, and was formalised as a first-order logic by Terry Halpin, see http://orm.net. ORM is thus the original user of the acronym. ORM and related modeling languages are distinguished by being attribute-free. These languages contain only objects and object types (kinds of things), facts and fact types (relationships between individual things) and constraints (rules about what things and relationships may exist). No relationship has the master-slave characteristic like entity-attribute - this is a notion that only arises during physical mapping, as it's irrelevant to the underlying semantics of the domain.
Object Relational Mapping (which I always write O/RM) is a name for a method or family of tools that help translate data between relational form and object-oriented form. Both these forms use aggregate or composite things based on attributes (entity/attribute or object/attribute), but the principles for aggregation differ between the two approaches, so the same underlying semantics results in different data structures; hence the need for tools to help automate the translation. Furthermore, in ER or O-O analysis, the need to make early decisions about which things are objects/entities and which are attributes is forced, and this gives rise to a whole class of modeling errors that simply does not occur with ORM.
Of course, both relational and o-o models can be automatically derived from an ORM model, and the mapping between the derived forms is also automatic and painless. I suppose that's not done more often because it would make life too easy.
You are comapring Apples to Oranges.
Object Relational Mapping is all about trying to overcome the impedance msimatch between the object world and relational databases.
Activerecord for example is a ORM that wraps a row in a database.
Hibernate is another popular ORM
Just google for ORM wikipedia explains it much better
http://en.wikipedia.org/wiki/Object-relational_impedance_mismatch
Related
I am developing Library Management System which have two sorts of books (Ebook and PrintedBook).
I intends to make search capacity with both ebook and printedbook in the same page.
The only problem is that I see that ebook and printedbook are book. And should I make an Book entity, and PrintedBook and Ebook inherits Book entity. If I do this, the search capacity is easier by using IBookRepository. If not I have to join two tables (Ebooks and PrintedBooks).
Please help me.
Dealing with inheritance at persistance level, esspecialy when talking about relation databases, can be a headache. First of all you should ask yourself why is this a problem for you.
If the problem is a performance due to using JOIN in you database query you might look at technique called single table inheritance. Basically you have one table containing all the columns of all your book types (i.e. PrintedBook and Ebook). This way you don't have to use JOIN, but you sacrifice some storage.
Other then the concrete table inheritanec technique (as described by yourself) there is no other way how to deal with the inheritance problem in relation databases.
If your application becomes too complex or the domain model isn't compatible with your read use cases, you might look at read-model. Read-model helps you to focus on your problem domain without modifying it while having easy access to the data. This is very complex topic so if you want to read something about read-models (or about DDD implementation problems/techniques) I recommend you to read Implementing Domain-Driven Design by Vaugh Vernon.
On SQLAlchemy's documentation page the author starts with a philosophy,
SQL databases behave less like object collections the more size and
performance start to matter; object collections behave less like
tables and rows the more abstraction starts to matter.
I'm scratching my head trying to understand the idea behind these two sentences, but failed. Could someone give an example illustrate the idea here? Thanks.
When you are creating an application using an Object Oriented language and a SQL database, you are simultaneously working with two very different conceptual models for storing information:
The relational model says how to store data in tables and rows and how to link elements through keys and joins.
The object model establishes a way to store entities with attributes in memory (usually) and how to set links between them using pointers or references.
So, let's say that you have an User entity that is linked to addresses and other users in your application. Those entities will need to be stored in the form several tables in the database (users table, addresses table and a many to many table for associating users to users, for instance). At the same time, if your code uses object oriented constructs, users and addresses will exist in memory in the form of objects with references between them, pointing to objects of the same or different kind.
The thing is, moving information between those two different worlds is much much more difficult than it looks at first:
You might associate one object with one row in a table, but that is not always possible and sometimes a single object must be associated to multiple rows in different tables.
Inheritance and polymorphic behavior are particularly difficult to map to a relational model.
Traversing objects and querying the database are vastly different actions.
Performance factors to take into account in an object model and a relational model are completely different.
And those are just a few examples. ORMs such as SQLAlchemy are essentially translators that convert information from one world into the other and back.
What I think that Mike Bayer was trying to convey is: the more you adapt your entity information to the object model (lots of inheritance, polymorphism, traversal of objects, ...), the farther it will resemble the natural structure in a relational model and the more performance concessions you will be making. And the other way around: the more you design your tables to perform well and be optimized for your queries, the less they will adapt to a natural structure of objects.
Martin Fowler has a nice write-up about the need of this translation in this article: ORM Hate (from which I took the above image).
Edit: further clarification on the abstraction vs performance issue
At the end, I think that the bottom line of that SQLAlchemy presentation text is: many ORMs hide the relational side of the relational-object oriented translation to make things easier. With them you only have to worry about the object oriented side, and the library is in charge of taking away the burden of dealing with the database. You get persistence for your objects without having to deal with SQL. However, they incur in a performance penalty in doing so, because the details of working with the database are abstracted away and you have no control over them. And those details are essential when you have to optimize performance. SQLAlchemy takes the opposite approach. It hides nothign of the relational side, you are in control of how SQL is generated and when and when not use joins, subqueries and other SQL constructs. That makes it a much more complex library to learn, but at the same time you are in control of the whole relational-object oriented translation process.
I am not sure if this is a good question as I'm unsure if there's any agreement on the subject. However due to the lack of information in the internet I'm compelled to ask anyways.
Let's say I'm making a system that is mainly object-oriented, with its corresponding UML diagrams (use-case, class, colaboration, etc). However, none of the UML diagrams are helpful when dealing with the database, which should be relevant for the developing team so they can know what exists in the database and what does not.
There are two ways to represent a database: Entity-Relationship and Relational (it's unknown to me if there are more, but those two are relevant within the relational database paradigm). ER deals with the representation of the BD in terms of business rules, and Relational deals with the actual, physical implementation. But none are "UML standard" (unless I'm missing something here).
Which modeling should I use, and why? Is ER relevant in terms of UML, or should I stick to relational? Thank you beforehand.
If you want to use UML only, you could use limited class diagrams - without m-n associations and methods. But if you are using some class-table mapping tools, you can use anything, except m-n relationships only.
Nobody had ever said that you can use Class diagram for OOP classes only. You can use them for any more or less formal concepts, if their needs can be covered by the complex of CD elements. I use class diagrams for UI planning and even formal text planning. And tables are very close to classes. So, no problem.
You can use data model diagrams, if you need something that is CALLED data diagram. But they are covered by class diagrams fully. That is the reason they are not supported anymore.
Your task is to make the model understandable for everybody, who can get it in hands. Class diagram is the most widely known UML diagram. A good title and a pair of comments will resolve all possible misunderstandings.
Both are different ER diagrams are relationship of entities and UML diagrams are behaviour of Ojects how they communicate with each other, as per my view point DFD (data flow diagram) is option. It has different levels which is based on number of processes and will better explain about data entities.
I know that UML is a general-purpose modeling language, but is it being used as a data model for describing a database like ER? Or is it being used only as a notation for entity relationship diagrams?
I mean: which data model tool would you use when you have to design a database for a company?
is the ER model still being the best choice?
Both can be regarded as equivalent when modeling database schemas. In fact, most database tools offer now both notations as alternatives when modeling the database.
I offer a more (personal) opinion on the differences between the two here but from a practical point of view use the one you prefer (and forget about a purist interpretation of their semantic differences)
In our e-commerce domain, we have a hierarchy of entities that are modeled using nested arrays. We do this using the principles of Domain-Driven Design (as explained by Eric Evans). The central concepts in our e-commerce domain are:
Contracts, which HAVE Exchanges, each of which HAS both Services and Payments. Services, in turn, HAVE Features that describe each service.
This hierarchical model enables us to express any contract, no matter how complex, including those that have multiple agreements (that is, Exchanges) as part of the overall agreement (or, Contract).
Does Drools not support such hierarchical object models? Should I invert my object model to a flat object model with no arrays (like the "Fires HAVE Rooms" & "Sprinklers HAVE Rooms" example in the Drools Expert documentation) as follows?
Contracts.
Exchanges, each of which HAS a single Contract.
Services and Payments, each of which HAS a single Exchange.
Features, each of which HAS a single Service.
Am I right that inverting hierarchical object models in this way, into flat object models with atomic assertions, is what is supported and works best in Drools? Drools doesn't appear to support a rule with LHS conditions on a fact and a fact that is in a subcollection.
If so, why doesn't Drools support more hierarchical object models? Is it because Drools comes from the AI world (not the object-oriented world) in which First-Order Logic expresses all facts as atomic subject-predicate-value statements, and not the object-oriented world in which entity objects have identity, value objects don't have identity, and entity objects are composed of other entity and value objects?
You can define rules against any Java object model.
The documentation provides examples based on toy problems to avoid distraction from what it's explaining. Not because Drools is incapable of dealing with more complex models. If you read a bit further through the manual, you will see examples for dealing with lists using syntax such as 'contains' or accumulators.
It's up to you how you model this. You can insert Contracts, Exchanges, Services, Payments and Features as separate facts, which reference each other. Alternatively, you could just insert a complex Contract fact, which contains lists of Exchanges, which contain lists of Services, etc.
Which works better for you depends on whether your rules match on a Contract with very little chaining, or whether you want a rule to react to something such as a change in a Feature, or the insertion of a Payment fact.
Nested accessors on objects is supported, as Drools works with any Pojo. However there is no reactivity on nested accessors.
It is possible, as a feature request, to start adding nested accessor reactivity, by inserting listeners. It's a non trivial piece of work, but it would be very interesting.