Previously when I wanted obtain related data in an sql query I would join tables, however now in linq to entities I want to get data from a table that is related to the table through another table. I don't know how to perform this sort of query in linq to entities. If someone could help that would be good.
The example is a table named person which has a relationship to table users which is related to table roles. I want to be able to obtain a person that has a particular role. Since person is only related to user and indirectly through user to role, I'm not sure of the query. Also using navigation properties doesn't get me all the way there either.
Any information would be good. Here is an example of the database structure:
db structure http://img194.imageshack.us/img194/4540/persons.jpg
If you using the generator in VS (ie, drap drop the data table and diagram and keys are all set in db), then the thing you are asking could be already there automatically.
e.g.
from Person in Context.Persons
where Person.Name == "PETER PAN"
select Person.User.Role.RoleName;
Exact name need to refer to the code generator, but this is the idea. Linq to entities will help to map foreign keys and those for you.
Edit
Actually I haven't tried using include. But according to msdn:include method, the include should show the object hierarchy to work. So, for your query to work, try:
from c in db.Persons.Include("aspnet_Users").Include("aspnet_Roles")
where c.aspnet_Users.aspnet_Roles.RoleName == "Role" select c
And moreover, will you consider start from roles?
from r in db.aspnet_Roles
where r.RoleName == "ROLE"
select r.aspnet_Users.Persons
(from u in db.aspnet_Users.Include("Person")
from c in db.aspnet_Roles
where c.RoleName == "role"
select u.Persons);
Worked it out thanks for trying though.
Related
I am using go and pq to interface with my postgres database.
I have a simple user table which has basic fields. Id, name, type. My auxillary table, admin inherits from user and adds it's own field panel, and another one that is owner and adds owner. Whether that be using table inheritance, or a supporting table.
My question is if I hit and endpoint that points to user/1 at this point I don't know what type of user this person is yet here. I know we can use jwts and other ways to provide this from the front end. I'm more curious about if there is a way to figure out the user and it's type and query the additional fields in one query?
Ie. I hit the endpoint I would Select from users, get the type, then use that type to get the additional fields. So I would effectively be doing two queries on two tables to get the complete data. Is there a better solution of doing this? Is there some optimizations I could do.
I have a legacy database with a particular table -- I will call it ItemTable -- that can have billions of rows of data. To overcome database restrictions, we have decided to split the table into "silos" whenever the number of rows reaches 100,000,000. So, ItemTable will exist, then a procedure will run in the middle of the night to check the number of rows. If numberOfRows is > 100,000,000 then silo1_ItemTable will be created. Any Items added to the database from now on will be added to silo1_ItemTable (until it grows to big, then silo2_ItemTable will exist...)
ItemTable and silo1_ItemTable can be mapped to the same Item entity because the table structures are identical, but I am not sure how to set this mapping up at runtime, or how to specify the table name for my queries. All inserts should be added to the latest siloX_ItemTable, and all Reads should be from a specified siloX_ItemTable.
I have a separate siloTracker table that will give me the table name to insert/read the data from, but I am not sure how I can use this with entity framework...
Thoughts?
You could try to use the Entity Inheritance to get this. So you have a base class which has all the fields mapped to ItemTable and then you have descendant classes that inherit from ItemTable entity and is mapped to the silo tables in the db. Every time you create a new silo you create a new entity mapped to that silo table.
[Table("ItemTable")]
public class Item
{
//All the fields in the table goes here
}
[Table("silo1_ItemTable")]
public class Silo1Item : Item
{
}
[Table("silo2_ItemTable")]
public class Silo2Item : Item
{
}
You can find more information on this here
Other option is to create a view that creates a union of all those table and map your entity to that view.
As mentioned in my comment, to solve this problem I am using the SQLQuery method that is exposed by DBSet. Since all my item tables have the exact same schema, I can use the SQLQuery to define my own query and I can pass in the name of the table to the query. Tested on my system and it is working well.
See this link for an explanation of running raw queries with entity framework:
EF raw query documentation
If anyone has a better way to solve my question, please leave a comment.
[UPDATE]
I agree that stored procedures are also a great option, but for some reason my management is very resistant to make any changes to our database. It is easier for me (and our customers) to put the sql in code and acknowledge the fact that there is raw sql. At least I can hide it from the other layers rather easily.
[/UPDATE]
Possible solution for this problem may be using context initialization with DbCompiledModel param:
var builder = new DbModelBuilder(DbModelBuilderVersion.V6_0);
builder.Configurations.Add(new EntityTypeConfiguration<EntityName>());
builder.Entity<EntityName>().ToTable("TableNameDefinedInRuntime");
var dynamicContext = new MyDbContext(builder.Build(context.Database.Connection).Compile());
For some reason in EF6 it fails on second table request, but mapping inside context looks correct on the moment of execution.
Is it possible to include a computed field in an EF entity? Example, lets say I have a shop selling products, and the products are grouped into categories.
When the list of categories is displayed for editing to the administrator of the shop I wish to list the number of products in each category as part of the list.
In NHibernate I would use a formula e.g.
<property name="Products" formula="dbo.Category_NumProducts(id)" />
I can't work out if something similar is possible for EF. I know I could make a category view and map this to a second entity but that seems wrong as its almost a complete duplication.
EDIT: If this isn't possible in EF, then what is the best way to accomplish this?
Unfortunately if your Category table doesn't have this as computed column you will not be able to map it without creating second entity - it leads to database view, defining query in EDMX or perhaps Query view in EDMX (but Query view may not work because you require aggregate function to be computed).
Edit:
IMHO the simplest solution (and also the best) is simply creating a ViewModel (some non mapped class) and use projection in Linq query:
var query = from c in context.Categories
where ...
select new CategoryView {
Id = c.Id,
Name = c.Name,
...
Products = c.Products.Count()
};
I have a dumb question. It would be great if this could be done, but I am not holding my breath.
I need a single column from a table linked to my JPA entity to be a collection in said JPA entity. Is there any way, that I can just get back that column alone that is related to that entity, instead of having to get back an entire table (which could be very costly?)
Can I perform a query inside that JPA entity that will be performed and loaded eagerly into a collection?
I am trying to avoid having to make several calls to the database by just executing a couple of queries.
What are your thoughts on this?
#ElementCollection(fetch=FetchType.EAGER)
#CollectionTable(name="QUICK_LAUNCH_DISTLIST",joinColumns=#JoinColumn(name="QUICK_LAUNCH_ID"))
#Column(name="LIST_ID")
private List<Long> distListIDs;
The ElementCollection attribute is what I was looking for. It seems to work pretty well in addition to that.
Thanks for the help and inspiration guys.
Suppose a Category has many products:
select product.name from Category c inner join c.products product where ...
If that's not what you want, please show an example in your question.
I was wondering with Entity Framework 4.1 code first how do you guys handle queries that involve an existing aspnet_Users table?
Basically I have a requirement for a query that involves the aspnet_Users so that I can return the username:
SELECT t.Prop1, u.Username
FROM Table1 t
INNER JOIN aspnet_User u ON t.UserId = u.UserId
Where t.Prop2 = true
Ideally in linq I would like:
from t in context.Table1
join u in context.aspnet_Users on t.UserId equals u.UserId
where t.Prop2 = true
But I'm not sure how to get aspnet_Users mapping to a class User? how do I make aspnet_Users part of my dbset ?
Any help would be appreciated, thanks in advance
Don't map aspnet_Users table or any other table related to aspnet. These tables have their own data access and their own logic for accessing. Mapping these tables will introduce code duplication, possible problems and breaks separation of concerns. If you need users for queries, create view with only needed information like id, user name, email and map the view. The point is that view will be read only, it will contain only allowed data and your application will not accidentally modify these data without using ASP.NET API.
First read Ladislav's answer. If you still want to go ahead : to do what you want would involve mapping the users and roles and members tables into the codefirst domain - which means writing a membership provider in code-first.
Luckily there is a project for that http://codefirstmembership.codeplex.com/ although its not a perfect implementation. The original is VB, look in the Discussion tab for my work on getting it running in c# MVC.
I'm working with the author on a better implementation that protects the membership data (password, last logged on date, all of the non-allowed data) but allow you to map and extend the user table. But its not ready yet!
You don't really need to use Entity Framework to access aspnet_membership provider accounts. You really just need to create an instance of the membership object, pass in a unique user identifier and a Boolean value indicating whether to update the LastActivityDate value for the user and the method returns a MembershipUser object populated with current values from the data source for the specified user.
You can then access the username by using the property of "Username".
Example:
private MembershipUser user =
Membership.GetUser(7578ec40-9e91-4458-b3d6-0a69dee82c6e, True);
Response.Write(user.UserName);
In case you have additional questions about MembershipProvider, you can read up on it on the MSDN website under the title of "Managing Users by Using Membership".
Hope this helps you some with your requirement.