I have a device table which defines the device entity:
device:
id:
type:
And I have a attribute table which records the device's attributes:
device_attr:
device_id:
key:
value:
How can I write the mappers to save the device POJO into the table and how to load the attributes into the POJO? thanks.
Pivot function may meet this, but, mysql doesnt support it, and it's performance is not good.
In order to save the object you can use dynamic sql to iterate over POJO properties and generate INSERT statements similar to what is described in How to use MyBatis to iterate all fields of an object? :
<bind name="deviceProperties"
value="#org.apache.commons.beanutils.BeanUtils#describe(myDevice).entrySet()" />
<foreach index="propertyName" item="propertyValue" collection="deviceProperties">
INSERT INTO device_attr (key, value) values (
${propertyName}, ${propertyValue});
</foreach>
There is no out of the box solution to do the same for the select.
You need to do what is called table pivot but this is very database specific.
Another option is to recreate the POJO from the list of fields in java. It can be done in mapper using default methods like described in this answer.
Related
I have an entity, for which apart from the primary key, an extra unique identifier should be generated:
#Entity
class MyEntity(
val otherId: String // <- this id is unique as well
) {
#Id
#Generated
var id: UUID // PK
}
otherId property value is derived from a postgres sequence value, by calling SELECT nextval(...) and adding a prefix string. When I do bulk inserts, I have to resort to a custom query defined in my JPA repository for the entity, which retrieves multiple sequence values at once, but I'd like to make this process automatic.
I tried to implement IdentifierGenerator interface, but the best I could achieve is a single SELECT nextval query was made for each new entity inserted, which is totally unacceptable in my case since batches can consist of hundreds of entities. Digging into the hibernate details didn't give me an answer how to do that either.
Is there a way to generate a number of ids via some callback/hook for multiple entities at once? Or I still have to do everything by hand?
There are hooks to implement this, see this article as an example: https://thorben-janssen.com/custom-sequence-based-idgenerator/
To improve performance, you will have to configure the increment size, which by default is 50. This means that it will increment the sequence by 50 and put these values in a pool from which the values are served for identity generation.
i would like to know if you have any idea how i can achieve this, considering a query stored as string in the configuration file.
I tried to use SqlQuery applied to the DBSet, but the problem is that SqlQuery requires me to select all properties of the required entities in my query. If i don't consider any column, it will complain because is not able to map the query to the entities.
I don't want to select all properties of the entities i want to query.
Thanks
If you are using EF then why not use Database.ExecuteSqlCommand()? It's in the System.Data.Entity namespace.
For example:
int result = db.Database.ExecuteSqlCommand("Non SELECT SQL etc...");
Well, I ended up implementing a mechanism using reflection that basically receives a group of fields to select, and constructs dynamic objects with those fields, so when applied the query with the joins between the entities, will only bring the fields I am looking for.
So, considering Entity1, Entity2, Entity3 with the following relationship
<b>Entity1</b>{
<br/> Entity1Name, <br/> List<*Entity2*> Entity2Items, <br/> etc..
<br/>}
and
<b>Entity2</b> { <br/> Entity2Name, <br/> List<*Entity3*> Entity3Items <br/>}
I can store e.g. the following query in the configuration file, and retrieve the information:
"Entity1.Entity1Name", <br/>
"Entity1.Entity2Items.Entity2Name", <br/>
"Entity1.Entity2Items.Entity3Items.Entity3Name"
Anyway, I was just trying to see if there would be any solution out-of-the-box that would require minimal code changes.
Thank you.
I know that it is possible using #SqlResultSetMapping but I want to select not whole entity from the database but some fields and then map i to my entity using one of the constructor which accept that fields. Is that possible to map result with #EntityResult for only a few #FieldResult? I was trying to do that and all the time I get error which said that there is not specify mapping for some fields which exist in that entity.
The disadvantage of #SqlResultSetMapping is that you have to select all the columns.
The alternate way of doing this manually iterate over the DB result and populate your objects.
Well, if you are using JPA 1.0 your only option (not considering the manual mapping, of course), is to use #SqlResultSetMapping and map the whole table columns. With JPA 2.1 you can add a javax.persistence.ConstructorResult (see docs here) to map only the needed columns.
When we have two entities in EFv4 EDM diagram and only one table for both in the database (for instance, having table Documents and entities Invoice and Qoute), table Documents having documentTypeId column as a discriminator and set this column as a discriminator in the EDM (in Table mappings), how do we read the value of this property in our code?
We cannot assign values to it because EF does it for us under the hood (based on what we entered in Table mappings for condition) but somehow I don't get it why we are also not allowed to read it.
Imo this property is already mapped so you can't map it again. It is used to determine type of materialized entity. Why do you need such column. Usually it is enough to use is operator like:
var document = context.Documents.GetById(id);
if (document is Invoice)
{
...
}
If you only need to select subtypes you can use OfType extension method like:
var invoices = context.Documents.OfType<Invoice>().ToList();
You also don't need to set this value when adding new entity because you are adding subtype - Invoice or Quote.
Edit:
As I understand from your comment you don't need this information in query. In such case you don't need to map it. Simply use partial class of your entity and add custom property which will return your string. Sound like stupid solution but actually it would be the easiest one.
Discriminator column should be part of mapping metadata so in case of T4 template generating your entities, it could be possible to update the template so it generate such property for you.
You may want to use a single-table inheritance hierarchy, as described here.
That way, you could have an abstract Document class that includes a DocumentTypeId column. Invoices and Quotes would extend this class, but specify certain DocumentTypeId filters. However, because the original class has a DocumentTypeId column, they would each have that column as well.
Another advantage to this approach is that you could create utility methods that can act on any Document, and you could pass any Invoice or Quote to these methods.
Is it possible to programmatically find out how core data matches a given class property to the database, i.e. in which table and which column the information will be stored?
Background: I would like to place an index on a specific column. I can find out the column by looking at the SQL core data executes. But there should be a more generic way to place the index, than hard-coding.
No, SQLite schema is a (private) implementation detail of CoreData. You can enable indexing of a property in the model editor in Xcode.