I want to have unique object ids over whole database. Is there any built-in feature in EF?
Or may be there are some practices/patterns?
thanks in advance.
If you are using SQL Server as the backend, you could look into using a UNIQUEIDENTIFIER column rather than an IDENTITY column, which means you can use GUIDs for your IDs across the entire database.
See "GUID Property Values" in the following MSDN article for EF4
http://msdn.microsoft.com/en-us/library/dd283139.aspx
1) Create table to store last id (int, bigint) (identifier domain)
2) Add stored procedure to retrieve the next id (int, bigint).
3) You may implement Custom Data Class EntityObject and ComplexObject (or maybe assign to every entity manualy in code)
4) Use this id in your custom class as primary key
--
If you using GUID...EF(4) has build-in feature to do this. But GUID as primary key with CLUSTERED INDEX is bad solution for the many insertion of course.
Related
How to refer another table built from composite type in my table.
I am trying to setup a database to understand postgresql and its Object oriented features.
The statement is as follows : There are multiple companies which can have board members.
Each company can own another company or a person can own that company too.
This is the type of database design I am looking for.
create type companyType(
name: VARCHAR,
boardMembers : personType[],
owns: companyType[]
)
create type personType(
name: VARCHAR,
owns: companyType[]
)
Create table company as companyType
Create table person as personType
I understand that I cannot self reference the companyType so I will probably move this another table.
My question is, when I am trying to insert into say company type, how do i insert list of person table objects as foreign key ?
Would making a column 'id' in each table and giving it type SERIAL work to use it as a foreign key?
That is not a relational database design, and you won't get happy with it.
Map each object to a table. The table columns are the attributes of the object. Add an artificial primary key (id bigint GENERATED ALWAYS AS IDENTITY). Don't use composite types or arrays.
Relationships are expressed like this:
If the relationship is one-to-many, add a foreign key to the "many' side.
If the relationship is many-to-many, add a "junction table" that has foreign keys to both tables. The primary key is the union of these foreign keys.
Normalize the resulting data model to remove redundancies.
Sprinkle with unique and check constraints as appropriate.
That way your queries will become simple, and you can use your database's features to make your life easier.
I went with PostgreSQL because it is an ORDMBS rather than a standard relational DBMS. I have a class/object (below) that I would like to implement into the database.
class User{
int id;
String name;
ArrayList<User> friends;
}
Now, a user has many friends, so, logically, the table should be declared like so:
CREATE TABLE user_table(
id INT,
name TEXT,
friends TYPEOF(user_table)[]
)
However, to my knowledge, it is not possible to use a row of a table as a type (-10 points for postgreSQL), so, instead, my array of friends is stored as integers:
CREATE TABLE user_table(
id INT,
name TEXT,
friends INT[]
)
This is an issue because elements of an array cannot reference - only the array itself can. Added to this, there seems to be no way to import the whole user (that is to say, the user and all the user's friends) without doing multiple queries.
Am I using postgreSQL wrong? It seems to me that the only efficient way to use it is by using a relational approach.
I want a cleaner object-oriented approach similar to that of Java.
I'm afraid you are indeed using PostgreSQL wrong, and possibly misunderstanding the purpose of Object-relational databases as opposed to classic relational databases. Both classes of database are still inherently relational, but the former provides allowances for inheritance and user-defined types that the latter does not.
This answer to one of your previous questions provides you with some great pointers to achieve what you're trying to do using the Postgres pattern.
Well, first off PostgreSQL absolutely supports arrays of complex types like you describe (although I don't think it has a TYPEOF operator). How would the declaration you describe work, though? You are trying to use the table type in the declaration of the table. If what you want is a composite type in an array (and I'm not really sure that it is) you would declare this in two steps:
CREATE TYPE ima_type AS ( some_id integer, some_val text);
CREATE TABLE ima_table
( some_other_id serial NOT NULL
, friendz ima_type []
)
;
That runs fine. You can also create arrays of table types, because every table definition is a type definition in Postgres.
However, in a relational database, a more traditional model would use two tables:
CREATE TABLE persons
( person_id serial NOT NULL PRIMARY KEY
, person_name text NOT NULL
)
;
CREATE TABLE friend_lookup
( person_id integer FOREIGN KEY REFERENCES persons
, friend_id integer FOREIGN KEY REFERENCES persons(person_id)
, CONSTRAINT uq_person_friend UNIQUE (person_id, friend_id)
)
;
Ignoring the fact that the persons table has absolutely no way to prevent duplicate persons (what about misspellings, middle initials, spacing, honorifics, etc?; also two different people can have the same name), this will do what you want and allow for a simple query that lists all friends.
I am trying to use Entity Framework DB first to do quick prototyping of a reporting website for a huge db. The problem is one of the tables doesn't have a key. I got an 'Error 159: EntityType has no key defined'. If I add a key on the model designer, I got 'Error 3024: Must specify mapping for all key properties'. My question is whether there is a way to workaround this WITHOUT adding a key to the table. The table is not in our control.
Huge table which does not have a key? It would not be possible for you or for table owner to search for anything in this table without using full table scan. Also, it is basically impossible to use UPDATE by single row without having primary key.
You really have to either create synthetic key, or ask owner to do that. As a workaround, you might be able to find some existing column (or 2-3 columns) which is unique enough that it can be used as unique key. If it is unique but does not have actual index created, that would be still not good for performance - you should create such index.
Using Open JPA 2.0, and database is DB2 9.7. I would like to like to create an entity which includes ID which is Primary Key, and auto generated Identity column and after creating the entity I need to display the ID generated. For this after persisting the entity, I am calling entity.getId().
In the database, I see below query getting executed
select ID from final table
(INSERT INTO WEB.USER (NAME, LOCATION) VALUES (?, ?))
Is there any alternate / better way to create an entity and return the generated ID? I would like to avoid "Select ID from final table", as this is expected to have performance impact.
This is how you retrieve auto generated ID from DB2 and there is nothing you can do about it. Other databases often require extra SQL query, so it's actually even better with DB2, which returns ID immedaitely. I don't think there is a faster way to so this.
Check out OPENJPA-736 where this optimization was implemented.
I am using Database sequence (#SequenceGenerator) to avoid "select ID from final table". This worked fine and #TableGenerator would also have sorted this issue.
I have 2 tables in sql server with primary keys set to identity. They are related and work fine.
I then created a form in vb 2008 and tried inserting some values into my database the respective primary keys work but the primary key in the parent table wont show up in the child table.I did create a relationship in vb using ado.net and all the details of my table are defineed in the data table. For example
cust tables (custid,name,..)
book table(bookid,bookname,..,custid)
in vb my insert statement is something like Insert into cust(name) values(#name)
insert into book(bookname) values(#bookname). I do not include the id columns as they auto generate in the database(tables).
My question is that how do i get to insert the custid in the book table when the data is stored back into the tavles in my database.
Please advice with an example as im not half as good as you guys.
Kind Regards
You have to know which customer you want to associate with the book before INSERTing the book. If you don't know before hand, you can't. So somewhere in your Form there should be a way to select a customer. Then when you create a book, you grab that customer's ID and insert it along with the other book info.
You don't actually say that you created a foreign key constraint between the two tables!
You need to:
Ensure that you create an explicit foreign key on the BOOK table to point to a customer in the CUST table.
First insert the customer.
Then find out what the customer's auto-generated ID was. That value is in ##IDENTITY. Store it somewhere e.g. #CUSTID.
Insert the book, specifying #CUSTID as the customer's ID.