Welcome. db4o is the native Java,.NET and Mono open source object database. This tutorial was written to get you started with db4o as quickly as possible. This documentation and tutorial is intended to get you started with db4o and to be a reliable The db4o object database engine consists of one single jar file. Introduction; Query by Example; Updates and identity; A search utility method; Advanced queries; Native queries in db4o; In conclusion.
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Queries, updates, and identity
Information storage and retrieval has been nearly synonymous with RDBMS for about a decade now, but recently that has begun to change. To close the transaction, use the Commit method:. A list of object ID matches tutoriall generated tugorial when the query is executed and held in memory. For example, including a console print statement System. This section shows you a solution based on using the Reflection APIs to poke the right values into the right fields, as well as suggesting ways to tune the solution for various preferences and aesthetics.
The third option is to create a query language that can then tktorial used to query the database’s object model. When using Query-By-Example, you create a prototypical object for db4o to use as an example of what you wish to retrieve.
The busy Java developer’s guide to db4o: Queries, updates, and identity
Matching values will be the same object instance as the supplied value. The default ActivationDepth in db4o is 5.
Welcome tuttorial is the native Java. The db4o object database engine consists tuutorial one single jar file.
The key thing to remember is that you can build convenience methods on top of the db4o infrastructure to make it easier to use. Global Update Depth We said earlier that when calling Store to update an object graph, that db4o will not by default descend into child objects to detect changes to the graph.
Ttorial tutorial was written to get you started with db4o as quickly as possible. The current version of db4o at tutorlal time of this writing is version 7. QBE lets you pass db4o an example object. Indexing fields is a great way to increase query performance, but each index table is one more place where a field’s value is stored. Developers were finally able to combine the power of a full database engine with plain undecorated objects. While you read through thistutorial it may be helpful to look into the API documentation occasionaly.
Then use the Solution Explorer to locate the new reference, right-click and open the properties window to ensure that “Copy Local” is set to true for each db4o assembly. Queries, updates, and identity. Closing a database cleanly will automatically call Commit for you, so any uncommitted transactions are committed automatically.
Using tables to store objects is like driving your car home and then disassembling it to put it into the garage. Whenever you start making changes to your database yutorial the Store and Dh4o commands you are automatically in an open transaction. QBE is the preferred mechanism for simple query situations because it’s an easier API to work with, but it does require that your domain objects permit any or all of the fields containing data to be set to null, which may violate some of your domain rules.
Thinking about this for a moment, you will probably begin to wonder how exactly this particular approach is being implemented. The db4o development team occasionally finds that certain APIs are less frequently used, or represent “experiments” on the API that the team isn’t sure should be a part of the core ObjectContainer API.
As you’ll learn, it’s actually quite amazing what you can do once you’re freed from the constraints of SQL. Over a million developers have joined DZone.
You can keep references to those constraints and then use constraint keywords like AndOrand Not to relate the constraints together, as we did at the end of that example. Best for servers and concurrent environments. Setting the UpdateDepth too aggressively can cause poor db4o update performance.
If you wish to cancel or roll back any uncommitted changes, use the Rollback method:. Rather than force a complex query API onto developers or introduce a new “something-QL,” db4o offers a facility called native querieswhich is both powerful and remarkably easy to use, as you can begin to see in Listing This content is part of the series: One difference, however, is that db4o will for performance reasons not automatically check child objects for changes.
Once you are familiar with storing objects, we encourage you to use Native Queries, the main db4o querying interface. Storing objects To store an object, we simply call set on our database, passing any object as a parameter. Combine with Smaller and Greater to include the specified value. Following that, I iterate through the array of objects that correspond to the values for each of those fields and then call Field.
In this particular case, let’s assume that the uniqueness of a Person in the system is its first name-last name combination.
Ttuorial db4o native query approach puts the compilation into bytecode back into the cb4o of the Java or C compiler, thus allowing for type safety and earlier detection of incorrect query syntax.
SODA query expressions are a standard that was present in all but the earliest versions of db4o. Within this method, I use the Reflection APIs on Class to create a new instance of that type invoking its default constructor. PDF for easy Reference.