In Entity Framework Code first i want to check database is exist before create Database.
In code first when i call Entities dc = new Entities() then it goes to OnModelCreating and generate Database. How can i check if the Database exists in Entity framework Code first?
You can do:
using(var dbContext = new MyContext())
{
if (!dbContext.Database.Exists())
dbContext.Database.Create();
}
Edit:
Following the colegue sugestion, the meaning of this code is very simple: Supose your context constructor is not set to create the database, so before sending any database operations, you can check if it exists, if not, you can create a new one with the connection string parameters being the rules for the creation.
This would be a static alternative, that works even without creating the DbContext first:
System.Data.Entity.Database.Exists(dbNameOrconnectionString);
Related
How can I get the insert_id (or existing id on duplicate key error) from a persist? I'd like to use it in following persists before flush.
In my mind, even though the EntityManager doesn't have an id yet, it could still populate that and use it during flush, but I think my thinking is flawed here, I'm new to MikroORM. From what I can see in the docs, I can only achieve this using the Query Builder.
Thanks for your help!
You can just build the entity graph, the entity does not need to have a PK to be used in a relation:
const book = new Book(); // new entity, no PK
user.favorites.push(book); // doesnt matter if it has PK
await em.flush(); // if `user` was a managed entity, this will find the new book and save it to the database
console.log(book.id); // now we have the PK available on entity
In other words, the entity instance is what holds the identity, you use that, not the PK.
Suppose that I have 2 aggregate roots (AR) in my domain and invoking some method on the 1st requires access to an instance of the 2nd. In DDD how and where should retrieval and creation of the 2nd AR happen?
Here's a contrived example TravelerEntity that needs access to a SuitcaseEntity. I'm looking for an answer that doesn't pollute the domain layer with infrastructure code.
public class TravelerEntity {
// null if traveler has no suitcase yet.
private String suitcaseId = ...;
...
// Returns an empty suitcase ready for packing. Caller
public SuitcaseEntity startTrip(SuitcaseRepository repo) {
SuitcaseEntity suitcase;
if (suitcaseId == null) {
suitcase = new SuitcaseFactory().create();
suitcase = repo.save(suitcase);
suitcaseId = suitcase.getId();
} else {
suitcase = repo.findOne(suitcaseId);
}
suitcase.emptyContents();
return suitcase;
}
}
An application layer service handling the start trip request would get the appropriate SuitcaseRepository implementation via DI, get the TravelerEntity via a TravelerRepository implementation and call its startTrip() method.
The only alternative I thought of was to move SuitcaseEntity management to a domain service, but I don't want to create the suitcase before starting the trip, and I don't want to end up with an anemic TravelerEntity.
I'm a little uncertain about one AR creating and saving another AR. Is this OK since the repo and factory encapsulate specifics about the 2nd AR? Is there a danger I'm missing? Is there a better alternative?
I'm new enough to DDD to question my thinking on this. And the other questions I found about ARs seem to focus on identifying them properly, not on managing their lifecycles in relation to one another.
Ideally TravelerEntity wouldn't manipulate a SuitcaseRepository because it shouldn't know about an external thing where suitcases are stored, only about its own internals. Instead, it could new up a SuitCase and add it to its internal [list of] suitcases. If you wanted that to work with ORMs without specifically adding the suitcase to the repository though, you'd have to store the whole suitcase object in TravelerEntity.suitcaseList and not just its ID, which conflicts with the "store references to other AR's as IDs" best practice.
Moreover, TravelerEntity.startTrip() returning a suitcase seems a bit artificial and unexplicit and you'll be in trouble if you need to return other entities created by startTrip(). So a good solution could be to have TravelerEntity emit a SuitcaseAdded event with the suitcase data in it once it has added the suitcase to its list. An application service could subscribe to the event, add the suitcase to SuitcaseRepository and commit the transaction, effectively saving both the new suitcase and the modified traveler to the database.
Alternatively, you could place startTrip() in a Domain Service instead of an Entity. There it might be more legit to use SuitcaseRepository since a domain service is allowed know about multiple domain entities and the overall domain process going on.
First of all persistence is not domain's job so i would get rid of all the repositories from the domain models and create a service that would use them.
Second of all you should rethink your design. Why a StartTrip method of a Traveller should return a SuitCase?
A Traveller either has or hasn't a suitcase. Once you have retrieved the Traveller you should already have their SuitCases too.
public class StartTripService {
public void StartTrip(int travellerId) {
var traveller = travellerRepo.Get(travellerId);
traveller.StartTrip();
}
}
Is there a way we can use ObjectContext with DbContext's ModelBuilder? We don't want to use POCO because we have customized property code that does not modify entire object in update, but only update modified properties. Also we have lots of serialisation and auditing code that uses EntityObject.
Since poco does create a proxy with EntityObject, we want our classes to be derived from EntityObject. We don't want proxy. We also heavily use CreateSourceQuery. The only problem is EDMX file and its big connection string syntax web.config.
Is there any way I can get rid of EDMX file? It will be useful as we can dynamically compile new class based on reverse engineering database.
I would also like to use DbContext with EntityObject instead of poco.
Internal Logic
Access Modified Properties in Save Changes which is available in ObjectStateEntry and Save them onto Audit with Old and New Values
Most of times we need to only check for Any condition on Navigation Property for example
User.EmailAddresses.CreateSourceQuery()
.Any( x=> x.EmailAddress == givenAddress);
Access Property Attributes, such as XmlIgnore etc, we rely heavily on attributes defined on the properties.
A proxy for a POCO is a dynamically created class which derives from (inherits) a POCO. It adds functionality previously found in EntityObject, namely lazy loading and change tracking, as long as a POCO meets requirements. A POCO or its proxy does not contain an EntityObject as the question suggests, but rather a proxy contains functionality of EntityObject. You cannot (AFAIK) use ModelBuilder with EntityObject derivatives and you cannot get to an underlying EntityObject from a POCO or a proxy, since there isn't one as such.
I don't know what features of ObjectContext does your existing serialisation and auditing code use, but you can get to ObjectContext from a DbContext by casting a DbContext to a IObjectContextAdapter and accessing IObjectContextAdapter.ObjectContext property.
EDIT:
1. Access Modified Properties in Save Changes which is available in ObjectStateEntry and Save them onto Audit with Old and New Values
You can achieve this with POCOs by using DbContext.ChangeTracker. First you call DbContext.ChangeTracker.DetectChanges to detect the changes (if you use proxies this is not needed, but can't hurt) and then you use DbCotnext.Entries.Where(e => e.State != EntityState.Unchanged && e.State != EntityState.Detached) to get DbEntityEntry list of changed entities for auditing. Each DbEntityEntry has OriginalValues and CurrentValues and the actual Entity is in property Entity.
You also have access to ObjectStateEntry, see below.
2. Most of times we need to only check for Any condition on Navigation Property for example:
User.EmailAddresses.CreateSourceQuery().Any( x=> x.EmailAddress == givenAddress);
You can use CreateSourceQuery() with DbContext by utilizing IObjectContextAdapter as described previously. When you have ObjectContext you can get to the source query for a related end like this:
public static class DbContextUtils
{
public static ObjectQuery<TMember> CreateSourceQuery<TEntity, TMember>(this IObjectContextAdapter adapter, TEntity entity, Expression<Func<TEntity, ICollection<TMember>>> memberSelector) where TMember : class
{
var objectStateManager = adapter.ObjectContext.ObjectStateManager;
var objectStateEntry = objectStateManager.GetObjectStateEntry(entity);
var relationshipManager = objectStateManager.GetRelationshipManager(entity);
var entityType = (EntityType)objectStateEntry.EntitySet.ElementType;
var navigationProperty = entityType.NavigationProperties[(memberSelector.Body as MemberExpression).Member.Name];
var relatedEnd = relationshipManager.GetRelatedEnd(navigationProperty.RelationshipType.FullName, navigationProperty.ToEndMember.Name);
return ((EntityCollection<TMember>)relatedEnd).CreateSourceQuery();
}
}
This method uses no dynamic code and is strongly typed since it uses expressions. You use it like this:
myDbContext.CreateSourceQuery(invoice, i => i.details);
I am trying to write a method to create a database and run migrations on it, given the connection string.
I need the multiple connections because I record an audit log in a separate database.
I get the connection strings out of app.config using code like
ConfigurationManager.ConnectionStrings["Master"].ConnectionString;
The code works with the first connection string defined in my app.config but not others, which leads me to think that somehow it is getting the connection string from app.config in some manner I don't know.
My code to create the database if it does not exist is
private static Context MyCreateContext(string ConnectionString)
{
// put the connection string where the factory method can get it
AppDomain.CurrentDomain.SetData("ConnectionString", ConnectionString );
var factory = new ContextFactory();
// I know I need this line - but I cant see how what follows actually uses it
Database.SetInitializer(new MigrateDatabaseToLatestVersion<Context,DataLayer.Migrations.Configuration>());
var context = factory.Create();
context.Database.CreateIfNotExists();
return context
}
The code in the Migrations.Configuration is
Public sealed class Configuration : DbMigrationsConfiguration<DataLayer.Context>
{
public Configuration()
{
AutomaticMigrationsEnabled = false;
}
}
The context factory code is
public class ContextFactory : IDbContextFactory<Context>
{
public Context Create()
{
var s = (string)AppDomain.CurrentDomain.GetData("ConnectionString");
return new Context(s);
}
}
Thus I am setting the connection string before creating the context.
Where can I be going wrong, given that the connection strings are all the same except the database name, and the migration code runs with one connection string, but doesnt run with others?
I wonder if my problem is to do with understanding how How does Database.SetInitializer actually works. I am guessing something about reflection or generics. How do i make the call to SetInitializer tie tie to my actual context?
I have tried the following code but the migrations do not run
private static Context MyCreateContext(string ConnectionString)
{
Database.SetInitializer(new MigrateDatabaseToLatestVersion<Context, DataLayer.Migrations.Configuration>());
var context = new Context(ConnectionString);
context.Database.CreateIfNotExists();
}
This question appears to be related
UPDATE:
I can get the migrations working if I refer to the connection string using
public MyContext() : base("MyContextConnection") - which points to in the config
I was also able to get migrations working on using different instances of the context, if I created a ContextFactory class and passed the connection to it by referencing a global. ( See my answer to the related question link )
Now I am wondering why it has to be so hard.
I'm not sure exactly as to what the problems are you facing, but let me try
The easiest way to provide connection - and be sure it works that way...
1) Use your 'DbContext' class name - and define a connection in the app.config (or web.config). That's easiest, you should have a connection there that matches your context class name,
2) If you put it into the DbContext via constructor - then be consistent and use that one. I'd also suggest to 'read' from config connections - and again name it 'the same' as your context class (use the connection 'name', not the actual string),
3) if none is present - EF/CF makes the 'default' one - based on your provider - and your context's class name - which usually isn't what you want,
You shouldn't customize with initializers for that reason -
initializers should be agnostic and serve other purpose - setup
connection in the .config - or directly on your DbContext
Also check this Entity Framework Code First - How do I tell my app to NOW use the production database once development is complete instead of creating a local db?
Always check 'where your data' goes - before doing anything.
For how the initializer actually works - check this other post of mine, I made a thorough example
How to create initializer to create and migrate mysql database?
Notes: (from the comments)
Connection shouldn't be very dynamic - config is the right place for it to be, unless you have a good reason.
Constructor should work fine too.
CreateDbIfNotExists doesn't work well together with the 'migration' initializer. You can just use the MigrateDatabaseToLatestVersion initializer. Don't 'mix' it
Or - put something like public MyContext() : base("MyContextConnection") - which points to <connectionStrings> in the config
To point to connection - just use its 'name' and put that into constructor.
Or use somehting like ConfigurationManager.ConnectionStrings["CommentsContext"].ConnectionString
Regarding entertaining 'multiple databases' with migrations (local and remote from one app) - not exactly related - but this link - Migration not working as I wish... Asp.net EntityFramework
Update:
(further discussion here - Is adding a class that inherits from something a violation of the solid principles if it changes the behavior of code?)
It is getting interesting here. I did manage to reproduce the problems you're facing actually. Here is a short breakdown on what I think it's happening:
First, this worked 'happily':
Database.SetInitializer(new CreateAndMigrateDatabaseInitializer<MyContext, MyProject.Migrations.Configuration>());
for (var flip = false; true; flip = !flip)
{
using (var db = new MyContext(flip ? "Name=MyContext" : "Name=OtherContext"))
{
// insert some records...
db.SaveChanges();
}
}
(I used custom initializer from my other post, which controls migration/creation 'manually')
That worked fine w/o an Initializer. Once I switched that on, I ran into some curious problems.
I deleted Db-s (two, for each connection). I expected to either not work, or create one db, then another in the next pass (like it did, w/o migrations, just 'Create' initializer).
What happened, to my surprise - is it actually created both databases on the first
pass ??
Then, being a curious person:), I put breakpoints on the MyContext ctor, and debugged through the migrator/initializer. Again empty/no db-s etc.
It created first instance on my call within the flip. Then on the first access to 'model', it invoked the initializer. Migrator took over (having had no db-s). During the migrator.Update(); it actually constructs the MyContext (I'm guessing via generic param in Configuration) - and calls the 'default' empty ctor. That had the 'other connection/name' by default - and creates the other Db all as well.
So, I think this explains what you're experiencing. And why you had to create the 'Factory' to support the Context creation. That seems to be the only way. And setting some 'AppDomain' wide 'connection string' (which you did well actually) which isn't 'overriden' by default ctor call.
Solution that I see is - you just need to run everything through factory - and 'flip' connections in there (no need for static connection, as long as your factory is a singleton.
You can supply a configuration in the MigrateDatabaseToLatestVersion constructor.
If you set the initializer in the DbContext you can also pass a 'true' to use the current connection string.
It could very well be that I'm just missing the correct vernacular in this space, but I'm looking for a particular piece of functionality in SubSonic. In NetTiers it was called a "DeepLoad". A deep load runs to the database and fetches many objects (ie. fetch this OrderDetail and all of it's LineItems) in one database call.
Again, I want to run to the data store once an build up a potentially dense object graph or related items populated by the data store.
How do I do this in SubSonic and what is it called in SubSonic?
You can do this in SubSonic 3.0 (not yet released, but almost there...) using IQueryable with lazy loading:
var db=new NorthwindDB();
var order=db.Orders.Where(x=>.xID==20).SingleOrDefault();
Assert.Equal(3,order.OrderDetails.Count());
if you're not on 3 (which requires .net 3.5) you can do this with Active record as Paul mentions - but it will make two calls.
There is no eager loading, and DeepSave in ActiveRecord only calls Save.
Here is an example with Northwind Order class foreign key method.
[Test]
public void SelectOrderDetails()
{
Order order = new Order(10250);
OrderDetailCollection details = order.OrderDetails();
Assert.IsTrue(details.Count == 3);
}