In my database(postgreSQl) is table Person which contains name and password. It is possible to dynamically(at Spring runtime) create new table by using Hibernate? For example I want to create something like single infoTable for every Person (when Person object is created). This table should have name like Person_Id+"infoTable". Is there any way to do that?
Hibernate does offer the mode called EntityMode.MAP that is excellent for unstructured data.
In order to use EntityMode.MAP, you want to create a Map<String, Object> structure where the map keys represent the column names and the object represents the value for the associated map key column. To save such data, you'd use:
session.save( "the_name_of_my_table", theEntityMap );
While this exists, I don't believe this is ideal for your use case though.
As others have suggested, you'd be better off creating an entity that contains a foreign key back to your Person entity and merely manage the multiple rows in a single table. There are numerous database features that can help easily deal with large volumes of data without having to resort to using EntityMode.MAP.
You set hibernate.hbm2ddl.auto=update in hibernate configuration, but think twice before doing so.
Related
CreateTableQuery = "Create Table XYZ";
s.createSQLQuery(createTableQuery.toString()).executeUpdate();
now I would like map a table with JPA and Hibernate? Its possible?
Tks.
JPA/Hibernate maps database tables to classes.
That means you would need a class that you can map the created table to.
If you already had this class then it's no longer dynamic.
For sure you could create a class at run-time using libraries and the try to generate the meta data of Hibernate. But I'm not sure if this is really your use case.
So my answer is NO it's not possible.
In one of the components of my application I have to deal with data which comes from a set of csv files (where content is added and updated over time) and store it into db to a very plain table with Hibernate. The data itself lacks of unique value which is suitable for use as a primary key, but the uniqueness could be achieved with a composite key instead. I'd like to get rid of composite key which was used as a temporary solution. Is there an idiomatic way to do this, keeping an entity objects behave the same way?
The first thing which came to my mind is some hash based solution, but I'm not sure about it.
Here's the case: I am creating a batch script that runs daily, parsing logfiles and exporting the data to a database. The format of this file is basically
std_prop1;std_prop2;std_prop3;[opt_prop1;[opt_prop2;[opt_prop3;[..]]]
The standard properties map to a table with a column for each property, where each line in the logfile basically maps to a corresponding row. It might look like LOGDATA(id,timestamp,systemId,methodName,callLenght). Since we should be able to log as many optional properties as we like, we cannot map them to the same table, since that would mean adding a row the table every time a new property was introduced. Not to think of the number of NULL references ...
So the additional properties go in another table, say EXTRA_PROPS(logdata_foreign_key,propname,value). In reality, most of the optional properties are the same (e.g. os version, app container, etc), making it somewhat wasteful to log for instance 4 rows in EXTRA_PROPS for each row in LOGDATA (in the case that one on average had 4 extra properties). So what I would like my batch job to do is
for each additionalProperty in logRow:
see if additionalProperty already exist
if exists:
create a reference to it in a reference table
if not:
add the property to the extra properties table
create a reference to it in a reference table
I would then probably have three slightly different tables:
LOGDATA(id,timestamp,systemId,methodName,callLenght)
EXTRA_PROPS(id,propname,value)
LOGDATA_HAS_EXTRA_PROPS(logid,extra_prop_id)
I am not 100% this is a better way of doing it, I would still create N rows in the LOGDATA_HAS_EXTRA_PROPS table for N properties, but at least I would not add any new rows to EXTRA_PROPS.
Even if this might not be the best way (what is?), I am still wondering about the tecnhical side: How would I implement this using Hibernate? It does not have to be superfast, but it would need to chew through 100K+ rows.
Firstly, I would not recommend using Hibernate for this type of logic. Hibernate is a great product but doing this kind of high load data operations may not be it's strongest point.
From data modeling standpoint, it appears to me that (propname,value) is actually a primary key in EXTRA_PROPS. Basically, you want to express the logic that, for example, hostname + foo.bar.com combination will only appear once in the table. Am I right? That would be PK. So you will need to use that in LOGDATA_HAS_EXTRA_PROPS. Using name alone will not be sufficient for reference.
In Hibernate (if you choose to use it), that can be expressed via composite key using #EmbeddedId or Embeddable on object mapped to EXTRA_PROPS. And then you can have many to many relationship that uses LOGDATA_HAS_EXTRA_PROPS as association table.
What is the recommended way in Java to add prefixes to String keys to be stored in database of a web application?
I have an EntityId per Entity but I want to store different kinds of data for an Entity, in different rows distinguished by prefixed EntityId keys like this format:
EntityId | PrefixForThisDataCategory
In general, databases resist that kind of thing. They're happy to store "prefix" characters in a separate column, though. If that column needs to be part of a composite key, they're happy to do that, too.
But if you want to store different kinds of data in different rows of the same table, I hope I can discourage you. Databases--SQL databases, that is--are designed to keep different kinds of data in different tables. People in one table, addresses in another table . . . not addresses in some rows of a table of people.
Of course, you might be aiming at something completely different.
If I understand your question correctly you basically want to store data representing different sets of information in a common table and use one or more fields to differentiate what type of data has been stored.
THIS IS A REALLY BAD IDEA ! - had to say it :-)
I can tell you from experience on many projects that storing data in this way always leads to problems and really messy code. My strong recommendation would be to store the data is separate tables.
There is one variation I can think of however, that is similar to your request, that is derived class mapping in hibernate. There hibernate maps sets of data into tables based on the class that is being stored. This is only for mapping hierarchies of classes and is controlled by hibernate so that you don't have to worry about it.
I'm getting introduced to serialization and ran into some problems when pairing it with LinkedList
Consider i have the following table:
CREATE TABLE JAVA_OBJECTS (
ID BIGINT NOT NULL UNIQUE AUTO_INCREMENT,
OBJ_NAME VARCHAR(50),
OBJ_VALUE BLOB
);
And i'm planning to store 3 object types - so the table may look like so -
ID OBJ_NAME OBJ_VALUE
============================
1 Class1 BLOB
2 Class2 BLOB
3 Class1 BLOB
4 Class3 BLOB
5 Class3 BLOB
And i'll use 3 different LinkedList's to manage these objects..
I've been able to implement LoadFromTable() and StoreIntoTable(Class1 obj1).
My question is - if i change an attribute for a Class2 object in LinkedList<Class2>, how do i effect the change in the DB for this individual item? Also take into account that the order of the elements in LinkedList may change..
Thanks : )
* EDIT
Yes, i understand that i'll have to delete/update a row in my DB table. But how do i keep track of WHICH row to update? I'm only storing the objects in the List, not their respective IDs in the table.
You'll have to store their IDs in the objects you are storing. However, I would suggest not trying to roll your own ORM system, and instead use something like Hibernate.
If you change an attribute in a an object or the order of items. You will have to delete that row and insert the updated list again.
How do i effect the change in the DB for this individual item?
I hope I get you right. The SQL update and delete statements allow you to add a WHERE clause in which you chose the ID of the row to update.
e.g.
UPDATE JAVA_OBJECTS SET OBJ_NAME ="new name" WHERE ID = 2
EDIT:
To prevent problems with your Ids you could wrap you object
class Wrapper {
int dbId;
Object obj;
}
And add them instead of the 'naked' object into your LinkedList
You can use AUTO_INCREMENT attribute for your table and then use the mysql_insert_id() function to retrieve the id assigned to the row added/updated by the last INSERT/UPDATE statement. Along with this maintain a map (eg a HashMap) from the java object to the Id. Using this map you can keep track of which row to delete/update.
Edit: See the answer to this question as well.
I think the real problem here is, that you mix and match different levels of abstraction. By storing serialized Java objects into a relational database as BLOBs you have to consider several drawbacks:
You loose interoperability. Applications written in other languages than Java are not able to read the data back. Even other Java applications have to have the class files of the serialized classes in their classpath.
Changing the class definitions of the stored classes will end up in maintenance nightmares.
You give up the advantages of a relational database. Serialization hides the actual data from the database. So the database is presented only with a black box. You are unable to execute any meaningfull query against the real data. All what you have is the ID and block of bytes.
You have to implement low level data handling by yourself. Actually the database is made to handle your data effectively, but because of serialization you hinder it doing its job. So you are on your own and you are running into that problem right now.
So in most cases you benifit from separation of concerns and using the right tool for a job.
Here are some suggestions:
Separate the internal data handling inside your application from persistent storage. Design your database schema in a way to enable the built-in database features to handle the data efficently. In case of a relational database like MySQL you can choose from different technologies like plain JDBC, object relational mappers like JPA or simple mappers like MyBatis. Separation here means to avoid to contaminate the database with implementation specific concerns.
If you have for example in your Java application a List of Person instances and each Person consists of a name and an age. Then you would represent that list in a relational database as a table consisting of a VARCHAR field for the name and a numeric field for the age and maybe a third field for a unique key. Then the database is able to do what it can do best: managing large amounts of data.
Inside your application you typically separate the persistent layer from the rest of your program containing the code to communicate with the database.
In some use cases a relational database may not be the appropiate tool. Maybe in a single user desktop application with a small set of data it may be the best to simply serialize your Person list into a plain file and read it back at the next start up.
But there exists other alternatives to persist your data. Maybe some kind of object oriented database is the right tool. In particular I have experiences with Fast Objects. As a simplification it is serialization on steroids. There is no need for a layer like JPA or JDBC between your application and your database. You are able to store the class instances directly into the database. But unlike the relational database with its BLOB field, the OODB knows your classes and the actual data and can benefit from that.
Another alternative may be JDBM or Berkeley DB.
So separation of concerns and choosing the right persistence strategy (and using it the right way) is a key concern for the success of your project. But doing it right is hard even for experienced developers.