I have created a mssql database and then used these instructions to create the datamodel. I have noticed something weird and it is also something which doesn't allow me to work with the model without confusion.
One of my objects is Book. Book contains an id, name, isbn, .. and also references to a Person table, like the author, the editor and the publisher.
the created model contains
string name;
int id;
string isbn;
int authorid;
int editorid;
int publisherid;
person person1;
person person2;
person person3;
Is there a way to change the names of the persons at creation? Because otherwise it is hard to figure out which person is who and I will have to change a lot manually.
Related
I want to create an entity containing 2 fields that need to be unique in together. One of the fields is a Boolean:
#Entity
public class SoldToCountry {
private String countryId;
private Boolean isInt;
}
For a given String there should never exist more than 2 entries one with isInt:true and the other isInt:false.
I read the doc about #Id but it seems that Boolean is not supported. For me it would also be ok to have a unique constraint spanned over both fields and using a generated id.
What is the best way to get this constraint via JPA?
If your table has really two fields only, and you want they are unique, then they should be the composite PK of the table. Take a look at How to create and handle composite primary key in JPA
If, instead, you have another PK, consider Sebastian's comment.
I have a Person table that I want to be mapped to multiple foreign keys in a Usage table. Something like this:
public class Usage{
int Id;
int OwnerId;
int CreatedBy;
public virtual Person Owner;
public virtual Person Creator;
}
public class Person{
int Id;
string Name;
}
Now I would like to use Fluent API so I can reference the properties Owner and Creator within an instance of the Usage table. Everything I read says you have to create relationships between both classes (e.g. also map a virtual collection back to the Usage table in the Person table) but I don't want to do that. I don't need a two way relationship. I just want to say that OwnerId and CreatedBy maps to the Person table and load it in memory.
modelBuilder.Entity<Usage>()
.WithOptional(x=>x.Owner).ForeignKey(x=>x.OwnerId)
.WithRequired(x=>x.Creator).ForeignKey(x=>x.CreatedBy)
Is this possible to do? I have a ton of relationships to the Person table and I don't want clutter it up with a bunch of virtual collections that I'll never use.
Thanks!
I am working on an intranet app. My data context class has some references to table(s) in other databases.
For example, one of my classes is like this:
public class SomeClass
{
public int Id {get;set;}
public string Name {get;set;}
...
public int EnteredById {get;set;}
}
Here is EnteredById is in fact a foreign key to Employee table in another database (on the same server) and that table is not part of the Data Context class.
I know I cannot create a foreign key on this property and this is fine as long as I can keep it in this class/ table and provide a value for it by going to Employee table in another database and retrieving it from there based on User’s Windows User Name.
How do I handle this situation while Employee table is not part of my Data Context?
I'm continuing exploring Cassandra and I would like to create Student <=> Course relation which is similar to Many-to-Many on RDBMS.
In term of Queries I will use the following query;
Retrieve all courses in which student enrolled.
Retrieve all students enrolled in specific course.
Let's say that I create to Column Families. one for Course and another for Student.
CREATE COLUMN FAMILY student with comparator = UTF8Type AND key_validation_class=UTF8Type and column_metadata=[
{column_name:firstname,validation_class:UTF8Type}
{column_name:lastname,validation_class:UTF8Type}
{column_name:gender,validation_class:UTF8Type}];
CREATE COLUMN FAMILY course with comparator = UTF8Type AND key_validation_class=UTF8Type and column_metadata=[
{column_name:name,validation_class:UTF8Type}
{column_name:description,validation_class:UTF8Type}
{column_name:lecturer,validation_class:UTF8Type}
{column_name:assistant,validation_class:UTF8Type}];
Now how should I move on?
Should I create third Column Family with courseID:studentId CompisiteKey? if yes, Can I use Hector to query by only one (left or right) Composite key component?
Please help.
Update:
Following the suggestion I created the following Schema:
For Student:
CREATE COLUMN FAMILY student with comparator = UTF8Type and key_validation_class=UTF8Type and default_validation_class=UTF8Type;
and then we will add some data:
set student['student.1']['firstName']='Danny'
set student['student.1']['lastName']='Lesnik'
set student['student.1']['course.1']=''
set student['student.1']['course.2']='';
Create column Family for Course:
CREATE COLUMN FAMILY course with comparator = UTF8Type and key_validation_class=UTF8Type and default_validation_class=UTF8Type;
add some data:
set course['course.1']['name'] ='History'
set course['course.1']['description'] ='History Course'
set course['course.1']['name'] ='Algebra'
set course['course.1']['description'] ='Algebra Course'
and Finally Student In Course:
CREATE COLUMN FAMILY StudentInCourse with comparator = UTF8Type and key_validation_class=UTF8Type and default_validation_class=UTF8Type;
add data:
set StudentInCourse['studentIncourse.1']['student.1'] ='';
set StudentInCourse['studentIncourse.2']['student.1'] ='';
I defined a data model below but it is easier to decribe the object model first and then dive into the row model, so from PlayOrm's perspective you would have
public class Student {
#NoSqlId
private String id;
private String firstName;
private String lastName;
#ManyToMany
private List<Course> courses = new ArrayList(); //constructing avoids nullpointers
}
public class Course {
#NoSqlId
private String id;
private String name;
private String description
#ManyToOne
private Lecturer lecturer;
#ManyToMany
private CursorToMany students = new CursorToManyImpl();
}
I could have used List in course but I was concerned I may get OutOfMemory if too many students take a course over years and years and years. NOW, let's jump to what PlayOrm does and you can do something similar if you like
A single student row would look like so
rowKey(the id in above entity) = firstName='dean',
lastName='hiller' courses.rowkey56=null, courses.78=null, courses.98=null, courses.101=null
This is the wide row where we have many columns with the name 'fieldname' and 'rowkey to actual course'
The Course row is a bit more interesting....because the user thinks loading al the Students for a single course could cause out of memory, he uses a cursor which only loads 500 at a time as you loop over it.
There are two rows backing the Course in this case that PlayOrm will have. Sooo, let's take our user row above and he was in course rowkey56 so let's describe that course
rowkey56 = name='coursename', description='somedesc', lecturer='rowkey89ToLecturer'
Then, there is another row in the some index table for the students(it is a very wide row so supports up to millions of students)
indexrowForrowkey56InCourse = student34.56, student39.56, student.23.56....
into the millions of students
If you want a course to have more than millions of students though, then you need to think about partitioning whether you use playOrm or not. PlayOrm does partitioning for you if you need though.
NOTE: If you don't know hibernate or JPA, when you load the above Student, it loads a proxy list so if you start looping over the courses, it then goes back to the noSQL store and loads the Courses so you don't have to ;).
In the case of Course, it loads a proxy Lecturer that is not filled in until you access a property field like lecturer.getName(). If you call lecturer.getId(), it doesn't need to load the lecturer since it already has that from the Course row.
EDIT(more detail): PlayOrm has 3 index tables Decimal(stores double, float, etc and BigDecimal), Integer(long, short, etc and BigInteger and boolean), and String index tables. When you use CursorToMany, it uses one of those tables depending on the FK type of key. It also uses those tables for it's Scalable-SQL language. The reason it uses a separate row on CursorToMany is just so clients don't get OutOfMemory on reading a row in as the toMany could have one million FK's in it in some cases. CursorToMany then reads in batches from that index row.
later,
Dean
I know there have been a number of similar posts about this, but I couldn't find a clear answer to my problem.
To make it as simple as possible, say I have such an entity:
#Entity
public class Person implements Serializable {
#Id
private Long id; // PK
private String name; // business key
/* getters and setters */
/*
override equals() and hashCode()
to use the **name** field
*/
}
So, id is the PK and name is the business key.
Say that I get a list of names, with possible duplicates, which I want to store.
If I simply create one object per name, and let JPA make it persistent, my final table will contain duplicate names - Not acceptable.
My question is what you think is the best approach, considering the alternatives I describe here below and (especially welcome) your own.
Possible solution 1: check the entity manager
Before creating a new person object, check if one with the same person name is already managed.
Problem: The entity manager can only be queried by PK. IS there any workaround Idon't know about?
Possible solution 2: find objects by query
Query query = em.createQuery("SELECT p FROM Person p WHERE p.name = ...");
List<Person> list = query.getResultList();
Questions: Should the objects requested be already loaded in the em, will this still fetch from database? If so, I suppose it would still be not very efficient if done very frequently, due to parsing the query?
Possible solution 3: keep a separate dictionary
This is possible because equals() and hashCode() are overridden to use the field name.
Map<String,Person> personDict = new HashMap<String,Person>();
for(String n : incomingNames) {
Person p = personDict.get(n);
if (p == null) {
p = new Person();
p.setName(n);
em.persist(p);
personDict.put(n,p);
}
// do something with it
}
Problem 1: Wasting memory for large collections, as this is essentially what the entity manager does (not quite though!)
Problem 2: Suppose that I have a more complex schema, and that after the initial writing my application gets closed, started again, and needs to re-load the database. If all tables are loaded explicitly into the em, then I can easily re-populate the dictionaries (one per entity), but if I use lazy fetch and/or cascade read, then it's not so easy.
I started recently with JPA (I use EclipseLink), so perhaps I am missing something fundamental here, because this issue seems to boil down to a very common usage pattern.
Please enlighten me!
The best solution which I can think of is pretty simple, use a Unique Constraint
#Entity
#UniqueConstraint(columnNames="name")
public class Person implements Serializable {
#Id
private Long id; // PK
private String name; // business key
}
The only way to ensure that the field can be used (correctly) as a key is to create a unique constraint on it. You can do this using #UniqueConstraint(columnNames="name") or using #Column(unique = true).
Upon trying to insert a duplicate key the EntityManager (actually, the DB) will throw an exception. This scenario is also true for a manually set primary key.
The only way to prevent the exception is to do a select on the key and check if it exists.