I am new to Entity Framework and LINQ and have run into a rather odd scenario.
I have been using the following query to return account information:
var account = ((from acct in _entities.Account
join m in _entities.Item on acct.Id equals m.Account.Id
where acct.Id == accountId && m.ItemNumber.EndsWith(itemNumber)
select acct) as ObjectQuery<Account>).Include("Item.ItemDetails");
We recently made some changes to the database and generated a new edmx file. Following the change the above query still returns the account and associated Item but the ItemDetails is no longer being included.
I have validated the SQL returned by the query and there doesn't seem to be anything wrong as the correct data is being returned.
Furthermore I don't see anthing different in the edmx file between the Item and ItemDetails objects as these were not changed and the navigation property is there.
Has anyone seen this before?
Thanks
In Include(...) is used the name of the navigation property so it will be good to check the exact name of the property from the .edmx (especially if it is singular or plural).
Also you can try to change the query like this:
var account = from acct in _entities.Account.Include("Item.ItemDetails")
join m in _entities.Item
on acct.Id equals m.Account.Id
where acct.Id == accountId && m.ItemNumber.EndsWith(itemNumber)
select acct;
You have one of two possible scenarios:
Item has a relationship to Account (expressed in your Entity Model as a EntityAssociation and in DB as a foreign key):
There is no relationship between Item set and Account set, hence, you must specify a join in LINQ as you have done.
Case 1: if this is the case, then you don't need a join statement... by selecting Acount.Item will naturally give you all items where Item.AccountID is equal to Account.ID
So your join statement: join m in _entities.Item on acct.Id equals m.Account.Id
has basically told Item to loop back onto Account to check the ID. If they were not already connected, then you could not have gotten m.Account.ID
Case 2: If there is no relationship between Account and Item, then the .Include() will definitely not work because the navigational property DOES NOT exist in your model.
Conclusion: Check your new model to see if a relationship exists between Account and Item. If yes, then remove the Join. If no relationship, then you've done something wrong.
Here is a select statement assuming scenario 1 and that Account.Item is not a collection:
var account = from acct in _entities.Account.Include("Item.ItemDetails")
where acct.Id == accountId && acct.Item.ItemNumber.EndsWith(itemNumber)
select acct;
Related
I have this table:
User
Name
Role
Mason
Engineer
Jackson
Engineer
Mason
Supervisor
Jackson
Supervisor
Graham
Engineer
Graham
Engineer
There can be exact duplicates (same Name/Role combination). Ignore comments about primary key.
I am writing a query that will give the distinct values from 'Name' column, with the corresponding 'Role'. To select the corresponding 'Role', if there is a 'Supervisor' role for a name, that record is returned. Otherwise, a record with the 'Engineer' role should be returned if it exists.
For the above table, the expected result is:
Name
Role
Mason
Supervisor
Jackson
Supervisor
Graham
Engineer
I tried ordering 'Role' in descending order, so that I can group by Name,Role and pick the first item - it will be a 'Supervisor' role if present, else 'Engineer' role - which matches my expecation.
I also tried doing User.select('DISTINCT ON (name) \*).order(Role: :desc) - I am not seeing this clause in the SQL query that gets executed.
Also, I tried another approach to get all valid Name, Role combinations and then process it offline iterating the result set and using if-else to decide which row to display.
However, I am interested in anything that is efficient and does not over do this handling.
I am new to Ruby and therefore reaching out.
If I wanted to do this in pure SQL, I would have to use GROUP BY.
SELECT Name, MAX(Role) FROM User GROUP BY Name
So one method would be to execute this SQL statement against the base connection.
ActiveRecord::Base.connection.execute("SELECT Name, MAX(Role) FROM User GROUP BY Name")
That would provide exactly the data you need, though it wouldn't be returned as ActiveRecord models. If you need those models then I would use find_by_sql and do an inner join to provide the records.
User.find_by_sql("SELECT User.* FROM User INNER JOIN (SELECT Name AS n, MAX(Role) AS r FROM User GROUP BY Name) U2 WHERE Name = U2.n AND Role = U2.r")
Unfortunately that would provide both records for Graham.
I have two models, Monkey and Session, where Monkey has_many Session. I have a scope for Monkey:
scope :with_session_counts, -> {
joins("LEFT OUTER JOIN `sessions` ON `sessions`.`monkey_id` = `monkeys`.`id`")
.group(:id)
.select("`monkeys`.*, COUNT(DISTINCT `sessions`.`id`) as session_count")
}
in order to grab the number of associated Sessions (even when 0).
Querying #monkeys = Monkey.with_session_counts works as expected. However, when I test in my view:
<% unless #monkeys.empty?%>
I get this error:
Mysql2::Error: Column 'id' in field list is ambiguous:
SELECT COUNT(*) AS count_all, id AS id FROM `monkeys`
LEFT OUTER JOIN `sessions` ON `sessions`.`monkey_id` = `monkeys`.`id`
GROUP BY `monkeys`.`id`
How would I convince Rails to prefix id with the table name in presence of the JOIN?
Or is there a better alternative for the OUTER JOIN?
This applies equally to calling #monkeys.count(:all). I'm using RoR 4.2.1.
Update:
I have a partial fix for my issue (specify group("monkeys.id") explicitly) I wonder whether this is a bug in the code that generates the SELECT clause for count(:all). Note that in both cases (group("monkeys.id") and group(:id)) the GROUP BY part is generated correctly (i.e. with monkeys.id), but in the latter case the SELECT only contains id AS id. The reason I say 'partial' is because it works in that it does not break a call to empty?, but a call to count(:all) returns a Hash {monkey_id => number_of_sessions} instead of the number of records.
Update 2:
I guess my real question is: How can I get the number of associated sessions for each monkey, so that for all intents and purposes I can work with the query result as with Monkey.all? I know about counter cache but would prefer not to use it.
I believe it is not a bug. Like you added on your update, you have to specify the table that the id column belongs to. In this case group('monkeys.id') would do it.
How would the code responsible for generating the statement know the table to use? Without the count worked fine because it adds points.* to the projection and that is the one used by group by. However, if you actually wanted to group by Sessions id, you would have to specify it anyway.
I have a Users table, a Roles table, and a cross reference table Users_Roles which has the following columns:
User_ID Role_ID Beamline_ID Facility_ID Laboratory_ID
The Beamline_ID, Facility_ID, Laboratory_ID are only filled in depending on the Role_ID. If someone has the Role_ID of 2 ("Lab Admin") then they will have an entry in Laboratory_ID.
I am trying to figure out how to get the Laboratory_ID for a specific row in this table. For example, I know I have User_ID = 1. I want to get the Laboratory_ID for User_ID = 1 and Role_ID = 2 ("Lab Admin").
This is obviously simple when dealing with SQL but I am new to Entity Framework and I am trying to do this with Entities and I'm having some trouble. I am using MVC so in my controller I have done this:
User user = new User();
user.GetUser(User.Identity.Name);
var labID = user.Users_Roles.Where(r => r.Role_ID == 2);
That should get me the "row" of that user when the Role = Lab Admin but I don't know how to grab the Labortory_ID column now. I thought maybe it would be:
var labID = user.Users_Roles.Where(r => r.Role_ID == 2).Select(l => l.Laboratory_ID);
But that is not correct. Any help would be greatly appreciated.
EDIT: I am using the database first approach and I am using DBContext. So typically I would access the context like this:
var context = new PASSEntities();
As for why the code you've already tried doesn't work, at first glance I think you just need to tack on a .Single() to the end:
var labID = user.Users_Roles.Where(r => r.Role_ID == 2)
.Select(l => l.Laboratory_ID)
.Single();
Select() returns a sequence, and it looks like you want a single value, so I think that might be the cause of the problem you were having. Single() will throw an exception if there's not exactly one result, which sounds appropriate for your table structure, but there's also First() if you don't care about enforcing that.
But for this kind of thing, you might also think about just querying manually using your DbContext, instead of trying to load entities individually and then traversing down through their navigation properties -- that results in more local searching than you probably need, and, depending on the circumstances, might be performing more/larger queries against your context and database than necessary too.
Here's an example LINQ query you could use (you might have to adjust the names of your tables and so on):
var labID = (from ur in context.Users_Roles
where ur.User_ID == 1 && ur.Role_ID == 2
select ur.Laboratory_ID).Single();
When you execute that statement, it will get translated into SQL equivalent to this:
SELECT TOP 1 Laboratory_ID FROM Users_Roles WHERE User_ID = 1 AND Role_ID = 2
...so it's pretty efficient. (Actually, if User_ID and Role_ID form the primary key for the Users_Roles table, and that record has already been loaded previously, I think it'll just return the cached copy, and won't need to query the database at all.)
That's a very bare-bones query, though; more than likely, you're eventually going to want to query based on properties of the user or role, instead of just searching for hard-coded IDs you know in advance. In that case, you can adjust your query to something like this:
var labID = (from u in context.Users
join ur in context.Users_Roles on u.User_ID equals ur.User_ID
join r in context.Roles on ur.Role_ID equals r.Role_ID
where u.UserName == "John Smith" && r.RoleName == "Lab Admin"
select ur.Laboratory_ID).Single();
That would, as you can probably tell, return the Lab ID for the user John Smith under the role Lab Admin.
Does that make any sense? LINQ syntax can take a little getting used to.
Lets say we have 3 tables, Users, Products, Purchases.
There is a view that needs to display the purchases made by a user.
I could lookup the data required by doing:
from p in DBSet<Purchases>.Include("User").Include("Product") select p;
However, I am concern that this may have a performance impact because it will retrieve the full objects.
Alternatively, I could select only the fields i need:
from p in DBSet<Purchases>.Include("User").Include("Product") select new SimplePurchaseInfo() { UserName = p.User.name, Userid = p.User.Id, ProductName = p.Product.Name ... etc };
So my question is:
Whats the best practice in doing this?
== EDIT
Thanks for all the replies.
[QUESTION 1]: I want to know whether all views should work with flat ViewModels with very specific data for that view, or should the ViewModels contain the entity objects.
Real example: User reviews Products
var query = from dr in productRepository.FindAllReviews()
where dr.User.UserId = 'userid'
select dr;
string sql = ((ObjectQuery)query).ToTraceString();
SELECT [Extent1].[ProductId] AS [ProductId],
[Extent1].[Comment] AS [Comment],
[Extent1].[CreatedTime] AS [CreatedTime],
[Extent1].[Id] AS [Id],
[Extent1].[Rating] AS [Rating],
[Extent1].[UserId] AS [UserId],
[Extent3].[CreatedTime] AS [CreatedTime1],
[Extent3].[CreatorId] AS [CreatorId],
[Extent3].[Description] AS [Description],
[Extent3].[Id] AS [Id1],
[Extent3].[Name] AS [Name],
[Extent3].[Price] AS [Price],
[Extent3].[Rating] AS [Rating1],
[Extent3].[ShopId] AS [ShopId],
[Extent3].[Thumbnail] AS [Thumbnail],
[Extent3].[Creator_UserId] AS [Creator_UserId],
[Extent4].[Comment] AS [Comment1],
[Extent4].[DateCreated] AS [DateCreated],
[Extent4].[DateLastActivity] AS [DateLastActivity],
[Extent4].[DateLastLogin] AS [DateLastLogin],
[Extent4].[DateLastPasswordChange] AS [DateLastPasswordChange],
[Extent4].[Email] AS [Email],
[Extent4].[Enabled] AS [Enabled],
[Extent4].[PasswordHash] AS [PasswordHash],
[Extent4].[PasswordSalt] AS [PasswordSalt],
[Extent4].[ScreenName] AS [ScreenName],
[Extent4].[Thumbnail] AS [Thumbnail1],
[Extent4].[UserId] AS [UserId1],
[Extent4].[UserName] AS [UserName]
FROM [ProductReviews] AS [Extent1]
INNER JOIN [Users] AS [Extent2] ON [Extent1].[UserId] = [Extent2].[UserId]
LEFT OUTER JOIN [Products] AS [Extent3] ON [Extent1].[ProductId] = [Extent3].[Id]
LEFT OUTER JOIN [Users] AS [Extent4] ON [Extent1].[UserId] = [Extent4].[UserId]
WHERE N'615005822' = [Extent2].[UserId]
or
from d in productRepository.FindAllProducts()
from dr in d.ProductReviews
where dr.User.UserId == 'userid'
orderby dr.CreatedTime
select new ProductReviewInfo()
{
product = new SimpleProductInfo() { Id = d.Id, Name = d.Name, Thumbnail = d.Thumbnail, Rating = d.Rating },
Rating = dr.Rating,
Comment = dr.Comment,
UserId = dr.UserId,
UserScreenName = dr.User.ScreenName,
UserThumbnail = dr.User.Thumbnail,
CreateTime = dr.CreatedTime
};
SELECT
[Extent1].[Id] AS [Id],
[Extent1].[Name] AS [Name],
[Extent1].[Thumbnail] AS [Thumbnail],
[Extent1].[Rating] AS [Rating],
[Extent2].[Rating] AS [Rating1],
[Extent2].[Comment] AS [Comment],
[Extent2].[UserId] AS [UserId],
[Extent4].[ScreenName] AS [ScreenName],
[Extent4].[Thumbnail] AS [Thumbnail1],
[Extent2].[CreatedTime] AS [CreatedTime]
FROM [Products] AS [Extent1]
INNER JOIN [ProductReviews] AS [Extent2] ON [Extent1].[Id] = [Extent2].[ProductId]
INNER JOIN [Users] AS [Extent3] ON [Extent2].[UserId] = [Extent3].[UserId]
LEFT OUTER JOIN [Users] AS [Extent4] ON [Extent2].[UserId] = [Extent4].[UserId]
WHERE N'userid' = [Extent3].[UserId]
ORDER BY [Extent2].[CreatedTime] ASC
[QUESTION 2]: Whats with the ugly outer joins?
In general, only retrieve what you need, but keep in mind to retrieve enough information so your application is not too chatty, so if you can batch a bunch of things together, do so, otherwise you'll pay network traffic cost everytime you need to go back to the database and retrieve some more stuffs.
In this case, assuming you will only need those info, I would go with the second approach (if that's what you really need).
Eager loading with .Include doesn't really play nice when you want filtering (or ordering for that matter).
That first query is basically this:
select p.*, u.*, p2.*
from products p
left outer join users u on p.userid = u.userid
left outer join purchases p2 on p.productid = p2.productid
where u.userid == #p1
Is that really what you want?
There is a view that needs to display the purchases made by a user.
Well then why are you including "Product"?
Shouldn't it just be:
from p in DBSet<Purchases>.Include("User") select p;
Your second query will error. You must project to an entity on the model, or an anonymous type - not a random class/DTO.
To be honest, the easiest and most well performing option in your current scenario is to query on the FK itself:
var purchasesForUser = DBSet<Purchases>.Where(x => x.UserId == userId);
That should produce:
select p.*
from products p
where p.UserId == #p1
The above query of course requires you to include the foreign keys in the model.
If you don't have the FK's in your model, then you'll need more LINQ-Entities trickery in the form of anonymous type projection.
Overall, don't go out looking to optimize. Create queries which align with the scenario/business requirement, then optimize if necessary - or look for alternatives to LINQ-Entities, such as stored procedures, views or compiled queries.
Remember: premature optimization is the root of all evil.
*EDIT - In response to Question Update *
[QUESTION 1]: I want to know whether all views should work with flat ViewModels with very specific data for that view, or should the ViewModels contain the entity objects.
Yes - ViewModel's should only contain what is required for that View. Otherwise why have the ViewModel? You may as well bind straight to the EF model. So, setup the ViewModel which only the fields it needs for the view.
[QUESTION 2]: What's with the ugly outer joins?
That is default behaviour for .Include. .Include always produces a left outer join.
I think the second query will throw exception because you can't map result to unmapped .NET type in Linq-to-entities. You have to return annonymous type and map it to your object in Linq-to-objects or you have to use some advanced concepts for projections - QueryView (projections in ESQL) or DefiningQuery (custom SQL query mapped to new readonly entity).
Generally it is more about design of your entities. If you select single small entity it is not a big difference to load it all instead of projection. If you are selecting list of entities you should consider projections - expecially if tables contains columns like nvarchar(max) or varbinar(max) which are not needed in your result!
Both create almost the same query: select from one table, with two inner joins. The only thing that changes from a database perspective is the amount of fields returned, but that shouldn't really matter that much.
I think here DRY wins from a performance hit (if it even exists): so my call is go for the first option.
HI I have a table with some some values (IDs), and of course when i get the result i got just the int IDs, but i want to put it more user friendly, for example when its the number 1, i want to put the string "Avaible", when its 2 "Not avaible", im on an N tiers enviroment and i need to get this done on the Model, whats the best way to accomplish this, i have to declare another class to project the strings, or must i use something like a dictionary, Key -> Value.
right now i just have this
return from t in db.products where t.productID==productID select t;
If you are using Linq to SQL you need another table to contain product status:
Table Name: Product Status
Fields: ProductStatusID int Indentity Primary Key
ProductStatus nvarchar(50)
Add a field to your Products Table:
Field to Add: ProductStatusID int
Add some statuses to your new table, and set the ProductStatusID of each product to an appropriate status id.
Add a constraint that connects the two ProductStatusID fields together. The easiest way do this is to create a diagram in SQL Server Management Studio Express, drag both tables onto the diagram, and then drag the ProductStatusID field from the ProductStatus table to the Products table, and click OK on the dialog that opens.
Rebuild your Linq to SQL data classes. You do this by deleting and recreating the DBML file, and dragging your tables into the designer again.
When you get a products object (p) from your dataContext object, you should now see this:
p.ProductStatus <-- The text description of the product's status.
Linq to SQL will reach into your ProductStatus table, and lookup the appropriate status description.