I have a .net core web api. Db is PostreSQL. I have a simple POST request that create an entity with two fields:
public class ClientDto{
public string Name {get;set;}
public int ClientId{get;set;}
}
ClientId - FK foreign key to table Clients.
Some client (Postman for exapmle) execute request, but in data model send ClientId that not exists in db.
I have global exeption handler and there I handle db exception, but exception object don't include separated information.
I would like to show to user beautiful message something like "Client with id = 1 not exists".
What the best practis to handle db exceptions?
May be before save object in db I need check if client with id = 1 exists in db? But it is an additional query.
May be before save object in db I need check if client with id = 1 exists in db? But it is an additional query.
I'd do this.
If your client doesn't give you good information in its exception then your probably better to do the additional query. If you're querying on an indexed field (which i'd expect given you are using a foreign key) then it will be a very quick query.
Exception throwing and catching is fairly expensive anyway and i'd probably be happy enough with the extra call.
Related
Given the following code, how can I add an element to one of the properties of an entity without knowing its Id and retrieving it from the database?
public async Task BookInPersonVisitAsync(Guid propertyId, DateTime dateTime, CancellationToken token)
{
var entity = new OnBoardingProcessEntity{ ExternalId = propertyId };
DbContext.OnBoardingProcesses.Attach(entity);
entity.OnBoardingProcessVisits.Add(new OnBoardingProcessVisitEntity
{
DateTime = dateTime,
Occurred = false
});
await DbContext.SaveChangesAsync(token);
}
ExternalId is just a guid we use for external reference. This doesnt work cause it does not have the id set, but without hitting the database we cant have it.
With entity framework if you have to reference an entity (referencedEntity) from another entity (entity) you have to know referencedEntity.
Otherwise you can add just add the entity setting the referencedEntity to null.
To know the referencedEntity or you know the Id or you have to retrieve it in some ways (from the database).
In SQL (DML) if (and only if) ExternalId is a candidate key noy nullable you can insert the OnBoardingProcessVisit record with a single roundtrip but the insert statement will contain an inner query.
OnBoardingProcessVisit.OnBoardingProcess_Id = (
SELECT
Id
FROM
OnBoardingProcess
WHERE
ExternalId = #propertyId)
EDIT
No way to generate that query with EF. You can have a look to external components (free and not free, for example EntityFramework Extended but in this case I think that doesn't help).
In this case I probably would try to use standard entity framework features (so 1 roundtrip to retrieve the OnBoardingProcess from the ExternalId).
Then, if the roundtrip is too slow, run the SQL query directly on the database.
About performances (and database consistency) add a unique index on OnBoardingProcess.ExternalId (in every case).
Another suggestion if you decide for the roundtrip.
In your code, the entity will be a proxy. If you don't disable lazy load, using your code you will do one more roundtrip when you will access to property
entity.OnBoardingProcessVisits (in the statement entity.OnBoardingProcessVisits.Add).
So, in this case, disable lazy load or do the same using a different way.
The different way in your case is something like
var onBoardingProcessVisitEntity new OnBoardingProcessVisitEntity
{
DateTime = dateTime,
Occurred = false,
OnBoardingProcess = entity
});
DbContext.OnBoardingProcessVisits.Add(onBoardingProcessVisitEntity);
await DbContext.SaveChangesAsync(token);
Kindly help me to find out a solution to my problem.
I have a property of type String in my entity which is not primary key.
This would act as system generated unique profile id for users in my project.
I want to auto generate this value like a random string of particular size.
Could you please help me to arrive at a solution.
I have tried the #Generator and custom id generator class. but still null is getting inserted to the field in db.
I am using maria db.
Hi you can use the hibernate events.
#PostPersist Executed after the entity manager persist operation is actually executed or cascaded. This call is invoked after the database INSERT is executed.
Starting with Java 5, the UUID class provides a simple means for generating unique ids. The identifiers generated by UUID are actually universally unique identifiers.
Please follow the sample example.
public class TestModel {
#Column("column_name")
private String uid;
//setter and getter for uid
#PrePersist
protected void onCreate() {
// set the uid
setUid(java.util.UUID.randomUUID());
}
}
Please find the below link for more information on #PrePersist.
https://docs.jboss.org/hibernate/entitymanager/3.5/reference/en/html/listeners.html
I've made a guestbook application using Google App Engine(GAE):python and the client is running on iPhone.
It has ability to write messages on the board with nickname.
The entity has 3 fileds:
nickname
date
message
And I'm about to make another feature that user can post reply(or comment) on a message.
But to do this, I think there should a 'primary key' to the guestbook entity, so I can put some information about the reply on a message.
With that three fields, I can't get just one message out of database.
I'm a newbie to database. Does database save some kind of index automatically? or is it has to be done by user?
And if it's done automatically by database itself(or not), how can I get just one entity with the key??
And I want to get some advise about how to make reply feature generally also. Thanks to read.
Every entity has a key. If you don't assign a key_name when you create the entity, part of the key is an automatically-assigned numeric ID. Properties other than long text fields are automatically indexed unless you specify otherwise.
To get an entity if you know the key, you simply do db.get(key). For the replies, you probably want to use a db.ReferenceProperty in the reply entity to point to the parent message; this will automatically create a backreference query in the message to get replies.
Each entity has a key, it contains information such as the kind of entity it is, it's namespace, parent entities, and the most importantly a unique identifier (optionally user specifiable).
You can get the key of an entity using the key method that all entities have.
message.key()
A key can be converted to and from a URL-safe string.
message_key = str(message.key())
message = Message.get(message_key)
If the key has a user-specified unique identifier (key name), you can access it like this
message.key().name()
Alternatively, if a key name was not specified, an id will be automatically assigned.
message.key().id()
To assign a key name to an entity, you must specify it when creating the entity, you are not able to add/remove or change the key name afterwards.
message = Message(key_name='someusefulstring', content='etc')
message.put()
You will then be able to fetch the message from the datastore using the key name
message = Message.get_by_key_name('someusefulstring')
Use the db.ReferenceProperty to store a reference to another entity (can be of any kind)
It's a good idea to use key name whenever possible, as fetching from the datastore is much faster using them, as it doesn't involve querying.
This is a follow-up to an earlier question I posted on EF4 entity keys with SQL Compact. SQL Compact doesn't allow server-generated identity keys, so I am left with creating my own keys as objects are added to the ObjectContext. My first choice would be an integer key, and the previous answer linked to a blog post that shows an extension method that uses the Max operator with a selector expression to find the next available key:
public static TResult NextId<TSource, TResult>(this ObjectSet<TSource> table, Expression<Func<TSource, TResult>> selector)
where TSource : class
{
TResult lastId = table.Any() ? table.Max(selector) : default(TResult);
if (lastId is int)
{
lastId = (TResult)(object)(((int)(object)lastId) + 1);
}
return lastId;
}
Here's my take on the extension method: It will work fine if the ObjectContext that I am working with has an unfiltered entity set. In that case, the ObjectContext will contain all rows from the data table, and I will get an accurate result. But if the entity set is the result of a query filter, the method will return the last entity key in the filtered entity set, which will not necessarily be the last key in the data table. So I think the extension method won't really work.
At this point, the obvious solution seems to be to simply use a GUID as the entity key. That way, I only need to call Guid.NewGuid() method to set the ID property before I add a new entity to my ObjectContext.
Here is my question: Is there a simple way of getting the last primary key in the data store from EF4 (without having to create a second ObjectContext for that purpose)? Any other reason not to take the easy way out and simply use a GUID? Thanks for your help.
I ended up going with a GUID.
The size/performance issues aren't
critical (or even noticeable) with SQL Compact, since
it is a local, single-user system.
It's not like the app will be
managing an airline reservation
system.
And at least at this point, there
seems to be no way around the "no
server-generated keys" limitation of
the SQL Compact/EF4 stack. If someone has a clever hack, I'm still open to it.
That doesn't mean I would take the same approach in SQL Server or SQL Express. I still have a definite preference for integer keys, and SQL Compact's bigger siblings allow them in conjunction with EF4.
Use a Guid. AutoIncrement is not supported on Compact Framework with Entity Framework.
Also, if you ever want to create a application which uses multiple data sources, int PK's are going to fall apart on you very, very quickly.
With Guid's, you can juse call Guid.NewGuid() to get a new key.
With int's, you have to hit the database to get a valid key.
If you store data in multiple databases, int PK's will cause conflicts.
What I've done for SQL CE before, and I assume we have a single application accessing the database, is to calculate the MAX value on startup and put it in a static variable. You can now hand out sequential values easily and you can make the code to generate them thread safe very easily.
One reason to avoid Guids would be size = memory and storage space consumption.
You could also query SQL Compact metadata like so:
SELECT AUTOINC_NEXT FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_NAME = 'Categories' AND AUTOINC_NEXT IS NOT NULL
I just started to play with the entity framework, so I decided to connect it to my existing SQL Server CE database. I have a table with an IDENTITY(1, 1) primary key but when I tried to add an entity, I've got the above-mentioned error.
From MS Technet artice I learned that
SQL Server Compact does not support entities with server-generated keys or values when it is used with the Entity Framework.
When using the Entity Framework, an entity’s keys may be marked as server generated. This enables the database to generate a value for the key on insertion or entity creation. Additionally, zero or more properties of an entity may be marked as server-generated values. For more information, see the Store Generated Pattern topic in the Entity Framework documentation.
SQL Server Compact does not support entities with server-generated keys or values when it is used with the Entity Framework, although the Entity Framework allows you to define entity types with server-generated keys or values. Data manipulation operation on an entity that has server-generated values throws a "Not supported" exception.
So now I have a few questions:
Why would you mark key as server-generated if it is not supported and will throw an exception? It's hard to make sence from the quoted paragraph.
When I've tried to add StoreGeneratedPattern="Identity" to my entity's property, Studio complained that it is not allowed. What I'm doing wrong?
What is the best workaround for this limitation (including switching to another DB)? My limitations are zero-installation and using entity framework.
When I hit this limitation, I changed the type to uniqueidentifier
Use uniqueidentifier or generate a bigint/int key value manually is your best option.
Something like this perhaps ...
private static object lockObject = new object();
private static long nextID = -1;
public static long GetNextID()
{
lock (lockObject)
{
if (nextID == -1) nextID = DateTime.UtcNow.Ticks; else nextID++;
return nextID;
}
}
This assumes that you don't generate more than one record per tick during an application run (plus the time to stop and restart). This is a reasonable assumption I believe, but if you want a totally bullet proof (but more complex) solution, go read the highest ID from the database and increment from that.
SQL CE version 4.0 fixed this problem with its Entity Framework provider.
I just hit this issue too... mostlytech's answer is probably the best option, GUIDs are very easy to use and the risk of key collision is very low (although not inexistant).
Why would you mark key as server-generated if it is not supported and will throw an exception? It's hard to make sence from the quoted paragraph.
Because SQL Server (not Compact) supports it, and other third parties may support it too... Entity Framework is not only for SQL Server Compact ;)
In my case, all of my classes have the primary key named "ID"
I created an interface
public class IID
{
public Int32 ID { get; set; }
}
Then I create an extension method
public static Int32 GetNextID<T>(this ObjectSet<T> objects)
where T : class, IID
{
T entry = objects.OrderByDescending(u => u.ID).FirstOrDefault();
if (entry == default(T))
return 1;
return entry.ID + 1;
}
Then when I need a new ID, I just do this:
MyObject myobj = new MyObject();
myobj.ID = entities.MyTable.GetNextID();
the other option is to use SqlCeResultSet on the tables that have the identity column.
i have a primary key named ID with data type of INT32 and have Identity Column
Just do this
MyEntity Entity = new MyEntity();
String Command;
command = "Insert into Message(Created,Message,MsgType)values('12/1/2014','Hello World',5);
Entity.ExecuteStoreCommand(command);
--Exclude the primary key in the insert Statement
--Since the SQLCE do not support system generated keys
--Do not use LINQ because it supplies a default value to 0 for Primary keys that has a
data type of INT