Using spring-data-neo4j, I want to create two classes using #RelationshipEntity(type="OWNS") to link a Person class to both a Pet and Car.
#RelationshipEntity(type="OWNS")
public class OwnsCar {
#Indexed
private String name;
#StartNode
private Person person;
#EndNode
private Car car;
}
#RelationshipEntity(type="OWNS")
public class OwnsPet {
#Indexed
private String name;
#EndNode
private Person person;
#StartNode
private Pet pet;
}
This saves to the Graph Database properly with no problems, as I can query the actual Node and Relationship and see they type, etc.
But when I attempt to use #RelatedTo(type="OWNS", elementClass=Pet.class) I either get a class cast exception, or when using lazy-initialization I get incorrect results.
#NodeEntity
public class Person {
#Indexed
private String name;
#RelatedTo(type="OWNS", direction=Direction.OUTGOING, elementClass=Pet.class)
private Set<Pet> pets;
#RelatedTo(type="OWNS", direction=Direction.OUTGOING, elementClass=Car.class)
private Set<Car> cars;
}
The result I get when I attempt to print our my person(my toString() has been omitted, but it simply calls the toString() for each field) is this:
Person [nodeId=1, name=Nick, pets=[Car [nodeId=3, name=Thunderbird]], cars=[Car [nodeId=3, name=Thunderbird]]]
Does anyone know if this can be done, should be done, is just a bug or a feature that is missing?
It seems like the problem is, that the annotation causes springDataNeo4j to priorize the relationship name. I tried the same on another sample I created. If both annotations contain
type="OWNS" it mixes both 'objects'. When I omit this information, and only use direction and type, it works for me.
Unfortunately this will lead to a problem if you are using another #RelatedTo annotation with more Pets or Cars related with another annotation. As it would not differ between "OWNS" and any other relation to a Pet-Type, the set returns all related pets (example: peter ->(HATES-Relationsip)->dogs).
If it's a bug or not, I can't tell... But for the database: There are only nodes and relations. Both are not typed, so neo4j does not know anything about your 'Pet'- or 'Car'-Class. Spring data neo4j handles this, either by indexing all nodes per type and setting a type-attribute, or using a specific graph-layout (with subreferences). Even if you would want to fetch all pets of a person with a traversal description, you would have so much more code to write, since the outgoing relations with name 'OWNS' contains two types of objects.
I would recommend using two different names. It's easier to write your custom traversals/queries later on, and its probably even faster as well, because no class-type comparison will be needed. Is there any reason, why you would need these specific names?
PS: It is possible, that not everything is 100% accurate. I don't know springdataneo4j in detail, but that's what I figured out so far.
Related
My schema will be something similar to the above picture.
I am planning to use Spring data JDBC and found that
If multiple aggregates reference the same entity, that entity can’t be part of those aggregates referencing it since it only can be part of exactly one aggregate.
Following are my questions:
How to create two different aggregates for the above without changing the DB design?
How to retrieve the Order / Vendor list alone? i.e. I don't want to traverse through the aggregate root.
How to create two different aggregates for the above without changing the DB design?
I think you simply have three Aggregates here: Order, Vendor and ProductType. A mental test that I always use is:
If A has a reference to B and I delete an A, should I automatically and without exception delete all Bs referenced by that A? If so B is part of the A Aggregate.
This doesn't seem to be true for any of the relationships in your diagram, so let's go with separate Aggregates for each entity.
This in turn makes each reference in the diagram one between different Aggregates.
As described in "Spring Data JDBC, References, and Aggregates" these must be modelled as ids in your Java code, not as Java references.
class Order {
#Id
Long orderid;
String name;
String description;
Instance created;
Long productTypeId;
}
class Vendor {
#Id
Long vid;
String name;
String description;
Instance created;
Long productTypeId;
}
class ProductType {
#Id
Long pid;
String name;
String description;
Instance created;
}
Since they are separate Aggregates each gets it's own Repository.
interface Orders extends CrudRepository<Order, Long>{
}
interface Vendors extends CrudRepository<Vendor, Long>{}
interface ProductTypes extends CrudRepository<ProductType, Long>{}
At this point I think we fulfilled your requirements. You might have to add some #Column and #Table annotations to get the exact names you want or provide a NamingStrategy.
You probably also want some kind of caching for the product types since I'd expect they see lots of reads with only few writes.
And of course you can add additional methods to the repositories, for example:
interface Orders extends CrudRepository<Order, Long>{
List<Orders> findByProductTypeId(Long productTypeId);
}
Good day fellow hibernators!
I have a question on how the DistinctBy clause works in conjunction with Spring Data's projection
Assume I have 3 classes:
public class Task {
Long id;
#ManyToOne(fetch = LAZY)
#JoinColumn(name = "project_id")
private Project project;
#OneToOne
#JoinColumn(name = "contact_id")
private Contact assigned;
Boolean deleted;
// ...
}
public class Contact {
Long id;
// ...
}
public class Project {
Long id;
#OneToMany(fetch = LAZY, mappedBy = "project")
private Set<Task> tasks;
// ...
}
These would be my domain classes. Notice, Project does have a "One2Many" to Tasks, Contact does not. Now, I have 2 interfaces for my projections and the basic TaskRepo with 2 methods:
public interface JustProject {
Project getProject();
}
public interface JustAssignee {
Contact getContact();
}
public class TaskRepo extends CrudRepository<Task, Long>, JpaSpecificationExecutor<Task> {
List<JustAssignee> findDistinctByDeletedFalse();
List<JustProject> findDistinctByDeletedFalseAndDeletedFalse();
}
The way it works for me right now is that, findDistinctByDeletedFalse returns as many instances as there are distinct contacts for tasks (e.g. if there are 10 tasks but only 3 contacts, the method will return just 3 objects containing all the 3 distinct contacts). Same for findDistinctByDeletedFalseAndDeletedFalse but on project level.
Now I have a few questions here and would love to get some help in understanding how this works exactly.
is the distinct clause applied after the search is done?
my initial assumption was that this behavior would not work as it does now. I assumed that the distinct clause is applied before the result is fetched, meaning that it would be DISTINCT based on the underlying task model, not the returned JustContact or JustProject model.
is there any way I could somehow not abuse the ...AndDeletedFalse redundant appendix? I need both the two methods from the repo but I feel like I had to cheat just to obtain that result...
... am I doing something wrong? I wanted to get "all distinct contacts/projects assigned to all tasks" as elegant of a way as possible. I ended up thinking about this distinctby exactly because I was unsure on how it works and wanted to try mu luck out. I really didn't think it would work this way, but now that it does I would really want to understand why it does!
Many thanks <3
The DISTINCT keyword is applied to the query and therefore it's effect depends on the select list which in turn is controlled by the projection. Therefore if you have only project or only contact in your projection the DISTINCT will get applied to those values only. Note though, that this relies somewhat on the boundaries of the JPA specification and I wouldn't be surprised if you see different behaviour with different implementations. See https://github.com/eclipse-ee4j/jpa-api/issues/189 and https://github.com/eclipse-ee4j/jpa-api/issues/124 for somewhat related issues raised against the specification.
In oder to differentiate methods that otherwise only differ in the return value you might add any additional string between find and By in the method name. For example you might want to rename your methods to findDistinctContactsByDeletedFalse and findDistinctProjectsByDeletedFalse
I guess this is the best that you can get with Spring Data JPA. You might be able to use just a single method by using the dynamic projections approach, but I think this is a perfect use case for Blaze-Persistence Entity Views.
I created the library to allow easy mapping between JPA models and custom interface or abstract class defined models, something like Spring Data Projections on steroids. The idea is that you define your target structure(domain model) the way you like and map attributes(getters) via JPQL expressions to the entity model.
A DTO model for your use case could look like the following with Blaze-Persistence Entity-Views:
#EntityView(Task.class)
public interface TaskAggregateDto {
// A synthetic "id" to get a grouping context on object level
#IdMapping("1")
int getGroupKey();
Set<ProjectDto> getProjects();
Set<ContactDto> getContacts();
#EntityView(Project.class)
interface ProjectDto {
#IdMapping
Long getId();
String getName();
}
#EntityView(Contact.class)
interface ContactDto {
#IdMapping
Long getId();
String getName();
}
}
The Spring Data integration allows you to use it almost like Spring Data Projections: https://persistence.blazebit.com/documentation/entity-view/manual/en_US/index.html#spring-data-features
public interface TaskRepo extends CrudRepository<Task, Long>, JpaSpecificationExecutor<Task> {
TaskAggregateDto findOneByDeletedFalse();
}
Lets say in my dao class i have a method annotated with sql: SELECT id, name, lat, long FROM table WHERE id = :id.
i want to map that to object like (pseudo):
public class Something {
public string Id;
public string Name;
public GeoLocation Location;
public Something(id, name, lat, long) {
this.Id = id;
this.Name = name;
this.Location = new GeoLocation(lat, long);
}
}
so, point is that i want to map flat select result into model with children made from some of the return fields.
Important is that i don’t want to have public get/set for all fields that sql returns.
I also want to avoid any room annotations on Something if possible (i am aware of solutions that involve #Embedded annotations).
In room the only possible way to convert fields into POJO that has no boiler point is indeed #Embedded annotation. Thats the best and simplest way to do it.
You can convert your sql query to your desired model with some other few methods which are not feasible.
Intermediate Model. That is convert your SQL result to a model that one to one matches to your fields. i.e.
public class PreSomething {
public string id;
public string name;
public long lat;
public long long;
}
After converting to PreSomething, you can have it get converted Something with any fashion you like.
Another way would be TypeConverters which changes database schema and will require you have database migration.
So, the only possible way is infact #Embedded. Now coming to your requirements,
Important is that i don’t want to have public get/set for all fields
that sql returns.
Every field that's stored in the database needs to be either public or have a "getter" method. Since your fields are all public you don't have to have any get/set for the fields.
I also want to avoid any room annotations on Something if possible (i
am aware of solutions that involve #Embedded annotations).
You have to annotate #Embedded the GeoLocation object (not Something) in order to be able to map your fields to a POJO.
One other thing to note that you SQLite, in that fashion Room, is case sensitive so. If you have to specify your fields with lowercase if your columns are lowercase. Otherwise you have to annotate them with #ColumnInfo and correct column name.
I have a User class, that is identified by id, and Skills class, that has its own id field, and also references User.
public class User {
#Id
#GeneratedValue
private int id;
#JsonIgnore
#OneToOne(mappedBy = "user")
private SoftSkills softSkills;
}
the other one has
#Entity
public class SoftSkills {
#Id
#GeneratedValue
private int id;
#OneToOne
#JoinColumn
private User user;
}
Is there a simple way to write a query, implementing the JPARepository, that would search the SoftSkills class by using user.id field as a parameter and return a SoftSkills object as a result?
You can, from the documentation:
Property expressions can refer only to a direct property of the managed entity, as shown in the preceding example. At query creation time you already make sure that the parsed property is a property of the managed domain class. However, you can also define constraints by traversing nested properties. Assume a Person has an Address with a ZipCode. In that case a method name of
List<Person> findByAddressZipCode(ZipCode zipCode);
creates the property traversal x.address.zipCode. The resolution algorithm starts with interpreting the entire part (AddressZipCode) as the property and checks the domain class for a property with that name (uncapitalized). If the algorithm succeeds it uses that property. If not, the algorithm splits up the source at the camel case parts from the right side into a head and a tail and tries to find the corresponding property, in our example, AddressZip and Code. If the algorithm finds a property with that head it takes the tail and continue building the tree down from there, splitting the tail up in the way just described. If the first split does not match, the algorithm move the split point to the left (Address, ZipCode) and continues.
So this will do the trick:
SoftSkills findByUserId(int id);
Reference; Spring Data JPA Documentation
I'm working on a Hibernate/Spring application to manage some movies.
The class movie has a many to many relationship with the class genre.
Both of these classes have generated id's using the GeneratedValue annotation.
The genre is saved through the movie object by using #Cascade(CascadeType.SAVE_UPDATE)
I have placed a unique constraint on the genre's type attribute (which is it's name; "Fantasy" for example).
What I would like to do now is have Hibernate check if there is already a genre with type "Fantasy" and if there is, use that genre's id instead of trying to insert a new record.
(The latter would obviously throw an error)
Finally what I need is something like select-before-update but more like select-before-save.
Is there such a function in Hibernate?
Some code:
Movie class
#Entity
public class Movie {
#Id
#GeneratedValue(strategy=GenerationType.AUTO)
private int id;
private String name;
#Lob
private String description;
#Temporal(TemporalType.TIME)
private Date releaseDate;
#ManyToMany
#Cascade(CascadeType.SAVE_UPDATE)
private Set<Genre> genres = new HashSet<Genre>();
.... //other methods
Genre class
#Entity
public class Genre {
#Column(unique=true)
private String type;
#Id
#GeneratedValue(strategy=GenerationType.AUTO)
private int id
....//other methods
You may be over-thinking this. Any select-before-update/select-before-save option is going to result in 2 DB round trips, the first for the select, and the second for the insert if necessary.
If you know you won't have a lot of genres from the outset, you do have a couple of options for doing this in 1 RT most of the time:
The Hibernate second-level cache can hold many if not all of your Genres, resulting in a simple hashtable lookup (assuming a single node) when you check for existence.
You can assume all of your genres are already existing, use session.load(), and handle the new insert as a result of the row not found exception that gets thrown when you reference a genre that doesn't already exist.
Realistically, though, unless you're talking about a LOT of transactions, a simple pre-query before save/update is not going to kill your performance.
I haven't heard of such a function in Hibernate select-before-update/select-before-save
In situations like these you should treat Hibernate as if it was JDBC.
First if you want to know if you even have such a Genre you should query for it.
if you do. then the SAVE_UPDATE will not create a new one when you add it to a movie.
if you don't, Hibernate will create a new Genre row in the database and add the connection to the many_to_many table for you.