Spring data : CrudRepository's save method and update - database

I wanted to know if the {save} method in CrudRepository do an update if it finds already the entry in the database like :
#Repository
public interface ProjectDAO extends CrudRepository<Project, Integer> {}
#Service
public class ProjectServiceImpl {
#Autowired private ProjectDAO pDAO;
public void save(Project p) { pDAO.save(p); } }
So if I call that method on an already registred entry, it'll update it if it finds a changed attribute ?
Thanks.

I wanted to know if the {save} method in CrudRepository do an update
if it finds already the entry in the database
The Spring documentation about it is not precise :
Saves a given entity. Use the returned instance for further operations
as the save operation might have changed the entity instance
completely.
But as the CrudRepository interface doesn't propose another method with an explicit naming for updating an entity, we may suppose that yes since CRUD is expected to do all CRUD operations (CREATE, READ, UPDATE, DELETE).
This supposition is confirmed by the implementation of the SimpleJpaRepository
class which is the default implementation of CrudRepository which shows that both cases are handled by the method :
#Transactional
public <S extends T> S save(S entity) {
if (entityInformation.isNew(entity)) {
em.persist(entity);
return entity;
} else {
return em.merge(entity);
}
}
So if I call that method on an already registered entry, it'll update
it if it finds a changed attribute?
It will do a merge operation in this case. So all fields are updated according to how the merging cascade and read-only option are set.

Looking at the default implemantation of CrudRepository interface
/*
* (non-Javadoc)
* #see org.springframework.data.repository.CrudRepository#save(java.lang.Object)
*/
#Transactional
public <S extends T> S save(S entity) {
if (entityInformation.isNew(entity)) {
em.persist(entity);
return entity;
} else {
return em.merge(entity);
}
}
Save method manage two situations:
-If the person Id is null (a new entity is created) then save will call persist method => insert query will be executed.
-If the person id is not null then save will call merge: fetch the existing entity from entityManagerFactory(from the 2 level cache if it doesn't exist then it will be fetched from the database) and comparing the detached entity with the managed and finally propagate the changes to the database by calling update query.

To be precise, the save(obj) method will treat obj as a new record if the id is empty (therefore will do an insert) and will treat obj as an existing record if the id is filled in (therefore will do the merge).
Why is this important?
Let's say the Project object contains an auto-generated id and also a person_id which must be unique. You make a Project object and fill in the person_id but not the id and then try to save. Hibernate will try to insert this record, since the id is empty, but if that person exists in the database already, you will get a duplicate key exception.
How to handle
Either do a findByPersonId(id) to check if the obj is in the db already, and get the id from that if it is found,
Or just try the save and catch the exception in which case you know it's in the db already and you need to get and set the id before saving.

I wanted to know if the {save} method in CrudRepository do an update if it finds already the entry in the database:
The Answer is Yes, It will update if it finds an entry:
From Spring Documentation: Herehttps://docs.spring.io/spring-data/jpa/docs/1.5.0.RELEASE/reference/html/jpa.repositories.html?
Saving an entity can be performed via the CrudRepository.save(…)-Method. It will persist or merge the given entity using the underlying JPA EntityManager. If the entity has not been persisted yet Spring Data JPA will save the entity via a call to the entityManager.persist(…)-Method, otherwise the entityManager.merge(…)-Method will be called.

In my case I had to add the id property to the Entity, and put the annotation #Id like this.
#Id
private String id;
This way when you get the object has the Id of the entity in the database, and does the Update operation instead of the Create.

Related

Dapper can't ignore nested objects for parameter?

I am beginning to use Dapper and love it so far. However as i venture further into complexity, i have ran into a big issue with it. The fact that you can pass an entire custom object as a parameter is great. However, when i add another custom object a a property, it no longer works as it tries to map the object as a SQL parameter. Is there any way to have it ignore custom objects that are properties of the main object being passed thru? Example below
public class CarMaker
{
public string Name { get; set; }
public Car Mycar { get; set; }
}
propery Name maps fine but property MyCar fails because it is a custom object. I will have to restructure my entire project if Dapper can't handle this which...well blows haha
Dapper extensions has a way to create custom maps, which allows you to ignore properties:
public class MyModelMapper : ClassMapper<MyModel>
{
public MyModelMapper()
{
//use a custom schema
Schema("not_dbo_schema");
//have a custom primary key
Map(x => x.ThePrimaryKey).Key(KeyType.Assigned);
//Use a different name property from database column
Map(x=> x.Foo).Column("Bar");
//Ignore this property entirely
Map(x=> x.SecretDataMan).Ignore();
//optional, map all other columns
AutoMap();
}
}
Here is a link
There is a much simpler solution to this problem.
If the property MyCar is not in the database, and it is probably not, then simple remove the {get;set;} and the "property" becomes a field and is automatically ignored by DapperExtensions. If you are actually storing this information in a database and it is a multi-valued property that is not serialized into a JSON or similar format, I think you are probably asking for complexity that you don't want. There is no sql equivalent of the object "Car", and the properties in your model must map to something that sql recognizes.
UPDATE:
If "Car" is part of a table in your database, then you can read it into the CarMaker object using Dapper's QueryMultiple.
I use it in this fashion:
dynamic reader = dbConnection.QueryMultiple("Request_s", param: new { id = id }, commandType: CommandType.StoredProcedure);
if (reader != null)
{
result = reader.Read<Models.Request>()[0] as Models.Request;
result.reviews = reader.Read<Models.Review>() as IEnumerable<Models.Review>;
}
The Request Class has a field as such:
public IEnumerable<Models.Review> reviews;
The stored procedure looks like this:
ALTER PROCEDURE [dbo].[Request_s]
(
#id int = null
)
AS
BEGIN
SELECT *
FROM [biospecimen].requests as bn
where bn.id=coalesce(#id, bn.id)
order by bn.id desc;
if #id is not null
begin
SELECT
*
FROM [biospecimen].reviews as bn
where bn.request_id = #id;
end
END
In the first read, Dapper ignores the field reviews, and in the second read, Dapper loads the information into the field. If a null set is returned, Dapper will load the field with a null set just like it will load the parent class with null contents.
The second select statement then reads the collection needed to complete the object, and Dapper stores the output as shown.
I have been implementing this in my Repository classes in situations where a target parent class has several child classes that are being displayed at the same time.
This prevents multiple trips to the database.
You can also use this approach when the target class is a child class and you need information about the parent class it is related to.

Add field to App Engine-hosted database

I'm currently developing a mobile application who uses a Google App Engine-hosted web service.
But i'm facing an issue. I just want to add a field in one my database's table.
App Engine doesn't use classic SQL syntax, but GQL. So i cannot use the ALTER TABLE statement. How can i do this with GQL ? I looked for a solution on the web, but there's not a lot of help.
public MyEntity() {
}
#Id
#GeneratedValue(strategy=GenerationType.IDENTITY)
private Key idStation;
private String name;
private double longitude;
private double latitude;
private java.util.Date dateRefresh = new Date(); //the field i want to add in DB
So, now when i create a "MyEntity" object, it should add the "dateRefresh" field into the database... I create my object like this:
MyEntity station = new MyEntity();
station.setName("test");
station.setLatitude(0);
station.setLongitude(0);
station.setDateRefresh(new Date("01/01/1980"));
DaoFactory.getStationDao().addStation(station);
addStation method:
#Override
public MyEntity addStation(MyEntity station) {
EntityManager em = PersistenceManager.getEntityManagerFactory().createEntityManager();
try {
em.getTransaction().begin();
em.persist(station);
em.getTransaction().commit();
} finally {
if(em.getTransaction().isActive()) em.getTransaction().rollback();
em.close();
}
return station;
}
The field "dateRefresh" is never created into my DB...
Someone to help me please ?
Thanks in advance
Just add another field to your data structure, maybe providing a default clause, and that's all. For example, if you have a UserAccount:
class UserAccount(db.Model):
user = db.UserProperty()
user_id = db.StringProperty()
you may easily add:
class UserAccount(db.Model):
user = db.UserProperty()
user_id = db.StringProperty()
extra_info = db.IntegerProperty(default=0)
timezone = db.StringProperty(default="UTC")
and let it go.
While the datastore kinda mimics tables, data is stored on a per entity basis. There is no schema or table.
All you need to do is update your model class, and new entities will be saved with the structure (fields) of the new entity.
Old entities and indexes, however, are not automatically updated. They still have the same fields as they had when they were originally written to the datastore.
There's two ways to do this. One is to make sure your code can handle situations where your new properties are missing, ie make sure no exceptions are thrown, or handle the exceptions properly when you're missing the properties.
The second way is to write a little function (usu a mapreduce function) to update every entity with appropriate or null values for your new properties.
Note that indexes are not updated unless the entity is written. So if you add a new indexed property, old entities won't show up when you query for the new property. In this case, you must use the second method and update all the entities in the datastore so that they are indexed.

Add a new attribute to entity in datastore?

I have an entity in my app engine datastore. There's actually only one instance of this entity. I can see it in my admin console. Is it possible to add a new attribute to the entity via the admin console (using gql perhaps)?
Right now it looks something like:
Entity: Foo
Attributes: mName, mAge, mScore
and I'd like to add a new boolean attribute to this entity like "mGraduated" or something like that.
In the worst case I can write some code to delete the entity then save a new one, but yeah was just wondering.
Thanks
-------- Update ---------
Tried adding the new attribute to my class (using java) and upon loading from the datastore I get the following:
java.lang.NullPointerException:
Datastore entity with kind Foo and key Foo(\"Foo\") has a null property named mGraduated.
This property is mapped to com.me.types.Foo.mGraduated, which cannot accept null values.
This is what my entity class looks like, I just added the new attribute (mGraduated), then deployed, then tried loading the single entity from the datastore (which produced the above exception):
#PersistenceCapable
public class Foo
{
#PrimaryKey
private String k;
/** Some old attributes, look like the following. */
#Persistent
#Extension(vendorName = "datanucleus", key = "gae.unindexed", value="true")
private String mName;
...
/** Tried adding the new one. */
#Persistent
#Extension(vendorName = "datanucleus", key = "gae.unindexed", value="true")
private boolean mGraduated;
The only way to implement this is to use Boolean as the type for the new property..
Than in set method you can accept boolean value, that's no issue.
If you want the get method to also return boolean.. you also can, but be sure to check if the value is null and if so.. return default value (e.g. true)
so
private Boolean newProp = null; // can also assing default value .. e.g. true;
public void setNewProp(boolean val)
{
this.newProp = val;
}
public boolean getNewProp()
{
if(this.newProp == null)
return true; // Default value if not set
return this.newProp.booleanValue();
}
I recommend you not to migrate your data in this case - it can be very costly and can deplete your quota easily (read old data, create new, delete old = 3 operations for every entry in you data store)
You can't do this through the admin console, but you shouldn't have to delete the entity. Instead just update it- the Datastore does not enforce schemas for Kinds.
E.g., if Foo is a subclass of db.Model (Python), change your model subclass to include the new property; fetch the model instance (e.g., by its key), update the instance, including setting the value of the new field; and save the modified instance. Since you just have one instance this is easy. With many such instances to update you'd probably want to do this via task queue tasks or via a mapreduce job.
You have declared the new mGraduated field using the primitive type boolean, which cannot be null. The existing entity can't be loaded into the model class because it doesn't have this property. One option is to declare this property using the Boolean class, which can accept a null value.
The Admin Console only knows about properties in existing entities. You cannot use the Admin Console directly to create a new property with a name not used by any existing entities. (This is just a limitation of the Console. App code can do this easily.)

How do I add a calculated column to my EF4 model?

Given a "User" table and a "Login" table in MS SQL 2008:
CREATE TABLE [dbo].[User_User](
[UserID] [int] IDENTITY(1000,1) NOT NULL,
[UserName] [varchar](63) NOT NULL,
[UserPassword] [varchar](63) NOT NULL
)
CREATE TABLE [dbo].[Util_Login](
[LoginID] [int] IDENTITY(1000,1) NOT NULL,
[User_UserID] [int] NOT NULL, -- FK REFERENCES [dbo].[User_User] ([UserID])
[LoginDate] [datetime] NOT NULL,
)
How do I adjust my User_User entity framework model object to include a "UserLastLogin" column that returns a MAX(LoginDate)?
I know that I can create an EF4 model around a SQL View:
CREATE VIEW [v_User_User]
AS
SELECT
[User_User].*,
(
SELECT MAX(LoginDate)
FROM [Util_Login]
WHERE User_UserID = UserID
) AS UserLastLogin
FROM [User_User]
But is there a way that I can just modify the User_User model to include the calculated columnn?
EDIT: I am looking for a way to fetch a User or a List<User> including the Max(Util.LastLogin) date in a single db query.
Very good question, and Yes, there is a perfect way to accomplish this in EF4:
Custom properties are a way to provide computed properties to entities. The good news is that Custom properties don’t necessarily need to be calculated from other existing properties on the very same entity, by the code we are about to see, they can computed from just about anything we like!
Here are the steps:
First create a partial class and define a custom property on it (For simplicity, I assumed User_User table has been mapped to User class and Util_Login to Util)
public partial class User {
public DateTime LastLoginDate { get; set; }
}
So, as you can see here, rather than creating a LastLoginDate property in the model, which would be required to map back to the data store, we have created the property in the partial class and then we have the option to populate it during object materialization or on demand if you don’t believe that every entity object will need to provide that information.
In your case precalculating the LastLoginDate custom property for every User being materialized is useful since I think this value will be accessed for all (or at least most) of the entities being materialized. Otherwise, you should consider calculating the property only as needed and not during object materialization.
For that, we are going to leverage ObjectContext.ObjectMaterialized Event which is raised anytime data is returned from a query since the ObjectContext is creating the entity objects from that data. ObjectMaterialized event is an Entity Framework 4 thing.
So all we need to do is to create an event handler and subscribe it to the ObjectMaterialized Event.
The best place to put this code (subscribing to the event) is inside the OnContextCreated Method. This method is called by the context object’s constructor and the constructor
overloads which is a partial method with no implementation, merely a method signature created by EF code generator.
Ok, now you need to create a partial class for your ObjectContext. (I assume the name is UsersAndLoginsEntities) and subscribe the event handler (I named it Context_ObjectMaterialized) to ObjectMaterialized Event.
public partial class UsersAndLoginsEntities {
partial void OnContextCreated() {
this.ObjectMaterialized += Context_ObjectMaterialized;
}
}
The last step (the real work) would be to implement this handler to actually populate the Custom Property for us, which in this case is very easy:
void Context_ObjectMaterialized(object sender, ObjectMaterializedEventArgs args)
{
if (args.Entity is User) {
User user = (User)args.Entity;
user.LastLoginDate = this.Utils
.Where(u => u.UserID == user.UserID)
.Max(u => u.LoginDate);
}
}
Hope this helps.
After much deliberation, I ended up with the following solution:
First, create a view containing all User fields plus a LastLogin date field (from my original post).
After adding the user (call it User_Model) and the user view (call it UserView_Model) to my EF model, I created a wrapper class (call it User_Wrapper) around the User_Model and added an additional DateTime property for LastLogin.
I modifed the User_Wrapper class to fetch from the UserView_Model, and then populate the underlying User_Model by reflecting over all the properties shared between the User_Model and UserView_Model. Finally, I set the User_Wrapper.LastLogin property based on the fetched User_View.
All other functions (Create,Update,Delete...) operate on the User_Model. Only the Fetch uses the UserView_Model.
What did all this do? I now only have one database call to populate a single User_Wrapper or a List<User_Wrapper>.
The drawbacks? I guess that because my UserView_Model does not have any associated relationships, I would not be able to do any eager loading using the EF ObjectContext. Fortunately, in my situation, I don't find that to be an issue.
Is there a better way?
I just had a situation where I needed count properties for two related entities without loading the collections. One thing I found out is that you need to have MultipleActiveResultSets=True in the connection string to avoid an exception being thrown on the ObjectMaterialized eventhandler when querying other entitycollections.

Need advice on complex insert scenario using LinqToSql with repository pattern

I have a User table and a ClubMember table in my database. There is a one-to-one mapping between users and club members, so every time I insert a ClubMember, I need to insert a User first. This is implemented with a foreign key on ClubMember (UserId REFERENCES User (Id)).
Over in my ASP.NET MVC app, I'm using LinqToSql and the Repository Pattern to handle my persistence logic. The way I currently have this implemented, my User and ClubMember transactions are handled by separate repository classes, each of which uses its own DataContext instance.
This works fine if there are no database errors, but I'm concerned that I'll be left with orphaned User records if any ClubMember insertions fail.
To solve this, I'm considering switching to a single DataContext, which I could load up with both inserts then call DataContext.SubmitChanges() only once. The problem with this, however, is that the Id for User is not assigned until the User is inserted into the database, and I can't insert a ClubMember until I know the UserId.
Questions:
Is it possible to insert the User into the database, obtain the Id, then insert the ClubMember, all as a single transaction (which can be rolled back if anything goes wrong with any part of the transaction)? If yes, how?
If not, is my only recourse to manually delete any orphaned User records that get created? Or is there a better way?
You can use System.Transactions.TransactionScope to perform this all in an atomic transaction, but if you are using different DataContext instances, it will result in a distributed transaction, which is probably not what you really want.
By the sounds of it, you're not really implementing the repository pattern correctly. A repository should not create its own DataContext (or connection object, or anything else) - these dependencies should be passed in via a constructor or public property. If you do this, you'll have no problem sharing the DataContext:
public class UserRepository
{
private MyDataContext context;
public UserRepository(MyDataContext context)
{
if (context == null)
throw new ArgumentNullException("context");
this.context = context;
}
public void Save(User user) { ... }
}
Use the same pattern for ClubMemberRepository (or whatever you call it), and this becomes trivial:
using (MyDataContext context = new MyDataContext())
{
UserRepository userRep = new UserRepository(context);
userRep.Save(user);
ClubMemberRepository memberRep = new ClubMemberRepository(context);
memberRep.Save(member);
context.SubmitChanges();
}
Of course, even this is a little bit iffy. If you have a foreign key in your database, then you shouldn't even need two repositories, because Linq to SQL manages the relationship. The code to create should simply look like this:
using (MyDataContext context = new MyDataContext())
{
User user = new User();
user.Name = "Bob";
user.ClubMember = new ClubMember();
user.ClubMember.Club = "Studio 54";
UserRepository userRep = new UserRepository(context);
userRep.Save(user);
context.SubmitChanges();
}
Don't fiddle with multiple repositories - let Linq to SQL handle the relationship for you, that's what ORMs are for.
Yes, you can do it in a single transaction. Use the TransactionScope object to begin and commit the transaction (and rollback if there is an error of course)

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