How can I track events on adding, updating or deleting entity rows with Linq2Db?
I need to recalculate some data in db on this operations, what is the best way?
On Entity Framework I use my custom Repository class with custom Add operations. Mabe this is only way in Linq2Db, but I am interesing, mabe there is some catcher to notify this events?
It is not so easy as in EF, because linq2db manipulates with SQL and you can easily update thousands records by single update statement.
For example
db.SomeEntities.Where(e => e.IsExpired)
.Set(e => e.ExpirationDate, e => Sql.CurrentTimestamp)
.Update();
The same technique can be used with insert from, update from, delete.
But there is possibility to catch SQL AST before executing. You have to override ProcessQuery method in your DataConnection class.
Sample is here: ConcurrencyCheckTests.cs
You should return the same statement that is passed to method but with changed property statement.IsParameterDependent = true to prevent query caching.
SqlStatement analysis is not trivial task but possible. You have to handle SqlUpdateStatement, SqlInsertStatement, SqlDeleteStatement, SqlInsertOrUpdateStatement.
Second option is to write your own extensions which manipulates with single objects, for example, UpdateTracked(), DeleteTracked(), etc. But as you can see from first example it will not help in complex queries.
Related
I would like to override the Query Async Method with some Custom code, so I can access an external api, get some data and then use this data to query the db tables to get the results, this should support all the default sync and paging features provided by the base method. I'm using the latest 5.0 DataSync library. Old versions returned a IQueryable so this was easy to do, but now it returns an action result. Is there any solution? Could not find any docs.
e.g. I get a set of guids from api. I query the db tables with these guids and get all the matching rows and return back to client.
I know how to override the method, call external api's, get data and query the db tables, but this provides me with a custom data format not the one that the query async gives by default with paging support.
The way to accomplish this is to create a new repository, swapping out the various CRUD elements with your own implementation. You can use the EntityTableRepository as an example. You will note that the "list" operation just returns an IQueryable, so you can do what you need to do and then return the updated IQueryable.
When creating an Axon JPA state-based aggregate is there a way to mark certain fields as being the #CreatedDate and #LastModifiedDate (as is possible with spring data jpa)?
In other words does Axon have the functionality where if any state of the aggregate is changed then axon automatically updates the #LastModifiedDate without us having to repeat it in every #CommandHandler?
Try using #CommandHandlerInterceptor inside your aggregate to intercept all commands and set
lastModifiedDate field.
#CommandHandlerInterceptor
public Object intercept(Object myCommand, InterceptorChain interceptorChain) throws Exception {
this.lastModifiedDate = Instant.now();
return interceptorChain.proceed();
}
I believe the proper solution would be to implement Axon's HandlerEnhancerDefinition interface to update these fields. This way you can grab the same timestamp (Instant) from the event that gets persisted in the event store and use that on your state-stored aggregate to make them match.
I wrote a blog post with a working example with a detailed explaination how to do this: https://michael.jeszenka.com/automatically-updating-timestamp-fields-for-axon-state-stored-aggregates/
Essentially, you will need to implement the wrapHandler() method to specify which types of event handlers you want to wrap with your enhancer. Then you will need to define a wrapper class to execute your desired behavior, which in our case is automatically handling the timestamps of the state-stored aggregate. This wrapper class will need to implement the Object handle(Message<?> message, T target) method which will allow us to grab the event timestamp from the meta data and use it to set the state-stored aggregate fields.
I'm trying to find the vocabulary to describe what I want and if it exists.
I have a table that shows a few data points from large objects. Loading the entire objects just for the table is very slow. Is there a way to only pass to the front the few properties I want without having to define a new object?
I found something called Sparse Fieldsets in JSON API, and I'm wondering if something like this exists for .NET under another name.
Update:
Talking to another coder, I realize it probably makes more sense to implement something like this between the backend and the database and make a specific call for this table. I still need to work out if I need to create a new object to support this. I think it'd still be faster if I just kept the same object but nulled out all of the connecting objects that I don't need for the table. But maybe that's considered bad practice? Also we're using Entity Framework for what it's worth.
Update 2:
I just created a new query without all of the .Include() and works well enough for what I need:
_dataContext.ApplePie
.Include(f => f.Apples).ThenInclude(f => f.Apple)
.Include(f => f.Sugars).ThenInclude(f => f.MolecularStructure)
.Include(f => f.Recipe)
Maybe you are looking for Anonymous Types?
For example, if you had a typed object with three properties, but you only wanted to operate on two:
var threePropThing = new ThreePropertyThing { Id = 1, Message = "test", ExtraProperty = "ex" };
var myAnonThing = new { Id = threePropThing.Id, Message = threePropThing.Message };
Best practice would be to not pass this anonymous object around. But, if you really needed to, you could return it as type object.
Typically, when passing data around in c#, you want to have it typed.
C# is a strongly-typed language and I would say that it is unusual for C# to support scenarios, when object definition (properties) are not known in advance, like in JSON API "fields" parameter case. Implementing this would imply using reflection to filter the properties, which is usually slow and error-prone.
When implementing C# web-services, people usually create one DTO response model per each request.
If your table has fixed set of fields, I would personally recommend to create a DTO class containing only the fields which are required for your table, and then create a method which returns this response for your specific request. While it doesn't align with "without having to define a new object" in the question, it makes the intention clear and makes it really easier to maintain the API in future.
You might want to use libraries like AutoMapper to save time and avoid duplicated code of copying the values from data model to DTO, if you have many such methods.
I am writing an application which communicates with an API and stores the response in a Meteor Collection so I can have the power of mongo to sort/filter.
I would like to clear the collection for every new result set. But a Meteor Collection is persistent.
What is the preferred way of clearing the collection? I know you can drop the meteor collection, but is that the preferred method?
Help appreciated. Thank you!
I would go about creating a local mongo collection which will be available on client side only. To create a client-side collection, just don't give it a name argument.
//This collection is client-only, and will not be sync with server
myCollection = new Mongo.Collection();
//To be more explicit, you can use `null` for the name:
myCollection = new Mongo.Collection(null);
Once you are done using the data empty the collection
myCollection.remove({});
myCollection.remove({}) is the syntax for removing all documents from a collection. This will only work on the server unless the collection is a client-side collection as per #Nakib's example. Otherwise documents can only be deleted by _id on the client side. Normally your allow/deny rules should block any attempt to delete anything on the client as it provides a great attack vector.
Not completely familiar with the Meteor best practice but if you were going to clear out an array in javascript the best practice would be to run the following.
myArrary.length = 0;
For more information I recommend this blog post by David Walsh where he details the reasoning behind zeroing out an array as follows:
Setting the length equal to zero empties the existing array, not
creating another array! This helps you to avoid pointer issues with
arrays as well.
I'm having a design issue in my project, related to where put some business logic.
I have three entities, Event, TicketOrder and Ticket. One Event has a lot of TicketOrders and one TicketOrder has a lot of Tickets.
In my template, I have to show how many tickets an Event has. I've thinking of the best approach to achieve this and didn't get a good solution. I've tried this:
1) Create a private member 'ticketsCount' in Event entity, with setTicketsCount and getTicketsCount method. Create a 'loadTicketsCount' method with a LifeCycleCallback 'PostLoad', to access the TicketRepository method 'findByEvent'. This was impossible because I can't access repository in an entity class.
2) In the action that will be used to display the Event, I can access Ticket Repository and set event 'ticketsCount' property manually. I don't know if it is a good approach because if my action is listing a lot of events I'll have to loop trough all events and make a repository call to each of then.
I really don't know the best approach to achieve this and will really appreciate if someone can help me.
Thanks! ;)
When you use findAll, findBy or findBy* methods of doctrine entity repository, a simple php array is returned containing the entity objects.
The array class implements countable interface. So using twigs length filter
{{ ticketOrder.tickets|length }}
you perform a simple php count() on the array.
Actually it makes now sense to perform a count query, because you already have the result in memory. So it seems more efficient to count the result and retrieve it from memory, because when you access associations they are completely loaded into memory.
However associations between entities can get pretty large. So imagine you have associations with hundred thousands of entities. You won't those entites to be loaded all together and kept in memory all the time. So in Doctrine 2.1 you can annotate an association as Extra Lazy. If you do so in your case a count query is performed when you call the above twig filter. But the result is not kept in memory.
http://docs.doctrine-project.org/en/2.0.x/tutorials/extra-lazy-associations.html
According to your latest comment:
I can imagine one way to do this. In a template you can call a controller's action with the render statement like
{% render YourMainBundle:getTickets with { 'event_id' : event.id } %}
and in this action you can call a query that looks for all tickets associated to the certain event. This action has to return html, e.g. an template filled with data.