问题
Which is better when using JPA, especially when starting a new project? Start with designing entities and then let JPA generate the database or Start with the database schema and let tools generate entity classes?
im part of a small company. im both the software developer and DBA. i have complete freedom for the application and db design
im just starting the project
回答1:
If you want to design a database, then start with the schema. If you want to write software, then start with the entities. The point of an ORM is to let you think about an object model without having to worry about the database that stores it, so questions of this type actually confuse the issue somewhat by insinuating crossover between the realms. Are you a software developer or a DBA? That, much more than the fact that you're using JPA, is what will determine the correct answer for you.
回答2:
Um - neither? The power of JPA is that you don't have to generate one from the other! Generation of entities or database schema might be a good starting place if you have one already in place; but the generated stuff is not something you will want to use long term.
You cannot simply design one side of the mapping without any consideration of the other. If you will share the database with other applications, you will need to give more weight to the database schema. If your application has a complex model, you will want to focus on the object model first, allowing it to be driven by the use cases you uncover as you develop your application.
I tend to start with the object model first (even without a backing database to start) because that allows me to see the application in action earlier and get a feel for what we really want to build. But integration with the database must happen earlier rather than later; as its constraints will quickly impose themselves on your object model. :-)
回答3:
It depends on one's needs . Usually in a product development environment , there are different teams which work on database design , interface design and implementation . So in that case , you have no option than generating the JPA entities from the already database design. Secondly , if you are starting from the scratch and you know what are you upto then you can probably start writing your own entities (java class) and then generate the database from that.
回答4:
Better go for database Scheme. Because some of the features r not available in JPA generated database.In JPA We cant give default values for our column. Check HERE for the allowable attributes in JPA.
回答5:
Its upto you. whether to go top-down or bottom up approach. In case this schema would be exclusive to your application, see how your team members and analysts understand ORM or DB. In my experience analysts are better understand in terms of tables. But if they are good to discuss in terms of classes or UML diagrams go with JPA. Also, take into consideration views of your DBA and build engineer.
回答6:
If you have complete flexibility and are not restricted to a DB schema and want a clean object model in your java application then start with with the model first and generate the schema from the model. This also allows you to generate clean JSON representations from the clean model to serve as on-the-wire format for your objects using technologies like Jackson (or GSON).
Doing DB Schema first and reverse engineering model classes from it will result in relational concepts seeping into your model classes resulting in poor (polluted) model.
In summary do model first unless your hands are tied and you must map to some existing schema.
来源：https://stackoverflow.com/questions/14887703/jpa-start-with-entities-vs-database-schema