How can I convert this expression to LINQ?
var result = users.FirstOrDefault(x => x.Name == userName)?
.Groups.FirstOrDefault(x => x.Group == userGroup);
I've started with:
var result = (from u in users
where u.Name == userName
select u).FirstOrDefault()?
My class is:
public class User
{
public string Name { get; set; }
public IEnumerable<Group> Groups { get; set; }
}
When creating this query, I don't have a separate groups list with which I can make a join on 2 tables.
But that's how far I managed to go. Is it possible to do a join within the same query?
I guess you want this:
var result = (from g in ((from u in users
where u.Name == userName
select u).FirstOrDefault().Groups)
where g == userGroup
select g).FirstOrDefault();
What you mean is how to convert method (or fluent) syntax to query (or comprehension) syntax. The first thing to note though is there's not one LINQ expression. The statement consists of two LINQ statements...
var user = users.FirstOrDefault(x => x.Name == userName);
var result = user?.Groups.FirstOrDefault(x => x.Group == userGroup);
...both of which can be written in query syntax, of which your starting point would be the first one.
However, the statement can be rewritten as one LINQ statement using SelectMany:
var result = users.Where(x => x.Name == userName)
.SelectMany(u => u.Groups.Where(g => g.Group == userGroup))
.FirstOrDefault();
This statement can be rewritten in one query-syntax statement:
var result = (from u in users
where u.Name == userName
from g in u.Groups
where g.Group == userGroup
select g).FirstOrDefault();
The advantage is that you don't need the null-propagation operator, which, by the way, you didn't apply sufficiently in your own statement.
One possible issue is that the results aren't necessarily identical. Originally you query a first user meeting a condition and of its groups a first group meeting another condition. The alternative query queries all users meeting a condition and from their groups the first one that meets another condition. So the first query may not return a result where the second does (if the matching group is not from the first user).
This may be an improvement or a flaw, I don't know. If the first condition uniquely identifies users it doesn't matter; the results will be the same. If it doesn't you may have to question its value because in a way it will return you a "random" user. You may want to use a lambda expression that narrows down the search to one specific user.
Related
I need to write some linq (linq-to-sql) for a search page that allows the user to search for cars and optionally include search criteria for the car's parts. The two tables are CAR and CAR_PARTS. Here is what I have so far:
var query = db.CAR;
//if the user provides a car name to search by, filter on car name (this works)
if(model.CarName != "")
{
query = from c in query
where c.Name == model.CarName
select c;
}
//if the user provides a car part name to filter on, join the CAR_PART table
if(model.CarPartName != "")
{
query = from c in query
join parts in db.CAR_PARTS on c.ID equals parts.CarID
where parts.PartName == model.CarPartName
select c;
}
//if the user provides a car part code to filter on, join the CAR_PART table
if(model.CarPartCode != "")
{
query = from c in query
join parts in db.CAR_PARTS on c.ID equals parts.CarID
where parts.PartCode == model.CarPartCode
select c;
}
If the user decides they want to search on both CarPartName and CarPartCode, this logic would result in the CAR_PART table being joined twice. This feels wrong to me, but is this the correct way to handle this?
How would you write this?
It's legal to do so, but whether it makes sense, depends on your datamodel and your desired outcome.
Generally your code does the following if partname and partcode are defined
Join the cars table with the parts table with partname as join condition
Join the result of the first join again with the parts table with partcode as join condition.
Thus, this is equal to a join with join condition car.partname = part.name and car.partcode = part.code. I don't know, whether this is your desired behaviour or not.
There are some cases to distinguish
Joining with AND condition
CASE 1.1: name and code of a part are keys in the parts table
In this case for each name and code are each unique in the parts table, thus for each name there is exactly one code. The double join is not necessary, and may even lead to wrong results, because
if selected name and code identify the same part, it's the first join will already get the desired results
if name and code identifiy different parts, your result will be empty because the condition cannot be fullfilled.
In that situation I would suggest to write is as follows
if (!string.IsNullOrEmpty(model.CarPartName)){
// your join on partname
} else if (!string.IsNullOrEmpty(model.CarPartCode)) {
// your join on partcode
}
CASE 1.2: name and code of a part are NOT keys in the parts table
In this case, neither name nor code may be unique, and for one name there may be different codes and vice versa. Here the double join is necessary and will only return results containing parts which match both, name and code
Joining with OR condition
If on the other hand you want your join condition to be like car.partname = part.name and car.partcode = part.code you have to consider the following cases
CASE 2.1 name and code are keys
Here applies the same as above in case 1.1
CASE 2.2 name and code are NOT keys
Here you can't use the stepwise approach, because the result of the first join will only contain cars, where the name matches. There may be parts where only the code condition matches, but they can never be included in the final result, if they are not contained in the result of the first match. So in this case, you will have to define your query something like this
if (!string.IsNullOrEmpty(model.CarPartName) && !string.IsNullOrEmpty(model.CarPartCode)) {
query = from c in query
join parts in db.CAR_PARTS on c.ID equals parts.CarID
where parts.PartName == model.CarPartName || parts.PartCode == model.CarPartCode
select c;
} else if (!string.IsNullOrEmpty(model.CarPartName)) {
query = from c in query
join parts in db.CAR_PARTS on c.ID equals parts.CarID
where parts.PartName == model.CarPartName
select c;
} else if (!string.IsNullOrEmpty(model.CarPartCode)) {
query = from c in query
join parts in db.CAR_PARTS on c.ID equals parts.CarID
where parts.PartCode == model.CarPartCode
select c;
}
What is wrong in there is actually with proper relations you don't need the join at all. Add that the behavior of LinqToSQL you can write that as:
var query = db.CAR
.Where( c =>
( string.IsNullOrEmpty(model.CarName)
|| c.Name == model.CarName ) &&
( string.IsNullOrEmpty(model.CarPartName)
|| c.Parts.Any( p => p.PartName == model.CarPartName )) &&
( string.IsNullOrEmpty(model.CarPartCode)
|| c.Parts.Any( p => p.PartCode == model.CarPartCode )));
Yours would work provided query is IQueryable (db.CAR.AsQueryable()). The two Linq approaches are similar but not the same. Depending on your real necessity yours might be the correct one or the wrong one. Yours would produce two inner joins, while this one simply create 2 exists check. Assume you have:
Car, Id:5, Name: Volvo
And parts like:
CarID:5, PartName:HeadLights, PartCode:1 ... other details
CarID:5, PartName:HeadLights, PartCode:2 ... other details
CarID:5, PartName:HeadLights, PartCode:3 ... other details
If user asks with model.CarName = "Volvo" and model.PartName = "HeadLights", you would get back the same Volvo 3 times. In second approach, you get back a single Volvo.
HTH
I feel more comfortable with fluent syntax, but I'm sure something similar to the following will work for you. I would check the fields in your model as part of a Select statement and then conditionally join using one field or the other. If neither are set, leave it null.
var query = db.CAR;
if (!string.IsNullOrWhitespace(model.CarName))
{
query = query.Where(car => car.Name == model.CarName);
}
var items = query.Select(car => new
{
Car = car, // maybe better to split this up into different fields, but I don't know what the car object looks like
// I assume your Car entity model has a navigation property to parts:
CarPart = !string.IsNullOrWhitespace(model.CarPartName)
? car.Parts.FirstOrDefault(part => part.PartName == model.CarPartName)
: !string.IsNullOrWhitespace(model.CarPartCode)
? car.Parts.FirstOrDefault(part => part.PartCode == model.CarPartCode)
: null
})
.ToList();
This does mean that the Code will be ignored if the Name is filled in. Reverse it if it needs to be the other way around. Or if you want to use both fields, you can put the string null checks in the Where clause.
I am writing a LINQ query that is performing a join on two tables, Artists and Groups (musical groups). The query is checking for artists that belong to a specific group and returning them as a list which I will iterate through and print to the console.
The problem is that I'm forced to write an else statement together with my if check and return something. I'm returning null currently, but want to skip that step entirely as the null values show up (as empty values) when I'm printing out my list. I've tried to use the .Distinct() method but it will still leave one null in my list.
var Beatles = Artists.Join(
Groups,
artist => artist.GroupId,
group => group.Id,
(artist, group) =>
{
if(artist.GroupId == 1)
{
return artist.ArtistName;
}
else{
return ;
}
})
.ToList()
.Distinct();
if(Beatles.Any())
{
System.Console.WriteLine("Here are all the members of the Beatles");
foreach(var person in Beatles)
{
System.Console.WriteLine(person);
}
};
You could simply use Where to filter.
var Beatles =
(from a in Artists
join g in Groups on a.GroupId equals g.Id
where a.GroupId == 1
select a.ArtistName).ToList();
And I couldn't figure out why are you applying join, it is completely redundant in this case.
var Beatles = Artists.Where(x => x.GroupId == 1).Select(x => x.ArtistName).ToList();
I've tried to use the .Distinct() method but it will still leave one
null in my list.
I assume that you use Distinct because of filtering the null values. But It is not purpose of Distinct. It is completely misusage.
I have this List:
string[] countries = {
"USA",
"CANADA"
};
When I run this query :
query = (from user in db where
user.Orders.Any(order => order.Price > 10 &&
countries.Contains(order.DestinationCountry)))
Output is a list of users that have Orders sent to "USA" OR "Canada".
but I want the list of users that have Orders sent to both "USA" AND" "CANADA".
I can do this using below code but i'm searching for a pure linq solution without any ForEach:
foreach (country in countries) {
query = (from user in query where
user.Orders.Any(order => order.Price > 10 &&
order.DestinationCountry == country));
}
Answers:
A. Using .Aggregate()
Generated query is just like For Each.
B.where countries.All(c => user.Orders.Any(o => o.Price > 10 && o.DestinationCountry == c))
When there is no element in Countries List (When I want all users based only on Price parameter), the result is not correct and other parameter is not considered!
Update 1:
I have tried .All() instead of .Contains() before posting and it returns 0 users.
Update 2:
I have updated my question to make it closer to the real problem.
lets say Country is not the only parameter.
Update 3:
Checked some answers and added the result to my question.
So you want a list of the users such that all the countries in the list are present in the set of order destinations?
Logically, that would be:
query = from user in db
where countries.All(c => user.Orders.Any(o => o.DestinationCountry == c))
select ...;
However, I'm not confident that EF will do what you want with that. It's not clear to me what the right SQL query would be to start with - in a simple way, at least.
query =
db.Users.Where(user =>
countries.All(country =>
user.Orders.Any(order =>
order.DestinationCountry == country)))
You can do it like this:
query = (from user in db where
user.Orders
.Where(o => countries.Contains(o.DestinationCountry))
.GroupBy(o => o.DestinationCountry)
.Count() == countries.Count
);
The idea is to keep only the orders going to countries of interest, then group by country, and check that the number of groups equals the number of countries.
It's possible using Enumerable.Aggregate:
query = countries.Aggregate(query,
(q, c) =>
from user in q
where user.Orders.Any(order => order.DestinationCountry == c)
select user);
but really, this is harder to understand than your foreach loop, so I'd just go with that.
Note that although I refer to a member of Enumerable, that member of Enumerable is actually building up an IQueryable<User> query chain just like your foreach loop, so this will not cause the filtering to move to the client.
I have the following setup: Tasks, Accounts and Groups tables. Tasks can be assigned to both individual accounts and groups. I've made two supporting tables: TasksAndAccounts and AccountsInGroups. TasksAndAccounts table has the following fields: TaskId, AccountId, GroupId and AccountsInGroups has AccountId and GroupId fields. I'm trying to write a query that will return all tasks assigned to a given account id. The culprit here is the query should look first in TasksAndAccounts table to see if there are tasks related directly to the given account and then also look for any groups that the given account is associated with.
The basic algorithm is simple:
Get all tasks from TasksAndAccounts where TasksAndAccounts.AccountId == accountId
Get all groups from AccountsInGroups where AccountsInGroups.AccountId == accountId
Get all tasks from TasksAndAccounts where TasksAndAccounts.GroupId is in the result set from step 2.
Merge steps 1 and 3.
I've tried to tackle the issue in a few different ways but wasn't able to get any satisfactory result.
Any ideas on writing an elegant single query solution?
This should translate into an EXISTS subquery:
var tasks = from task in dc.TasksAndAccounts
where task.AccountId == accountId || (
from g in dc.AccountsInGroups
where g.AccountId == accountId && g.GroupId == task.GroupId
select g.GroupId
).Any()
select task;
Ugh. Looks like you'll need a sub-select.
var ids = from task in context.TaskAndAccounts
where task.AccountId == accountId ||
(from grp in context.AccountsInGroups
where grp.AccountId == accountId
select grp.GroupId).Contains(task.GroupId)
select task;
Personally, I'd do it something like this:
var tasks = db.TasksAndAccounts.Where(x => x.AccountId == accountId);
var groups = db.AccountsInGroups.Where(x => x.AccountId == accountId);
var groupIDs = groups.Select(x => x.GroupId);
var groupTasks = db.TasksAndAccounts.Where(x => groupIDs.Contains(x.GroupId));
var allTasks = tasks.Union(groupTasks);
It's more than one line, but it's a lot clearer than trying to cram the whole thing into one line, in my opinion. Since LINQ uses deferred execution, you still won't be executing anything until you're actually using the allTasks result set.
Should be something like:
var tasks = from taa in TasksAndAccounts.Where(t => t.AccountId == accountId)
join aig in AccountsInGroups.Where(a => a.AccountId == accountId) on taa.GroupId equals aig.GroupId
select taa;
I'm wondering if its possible to join together IEnumerable's.
Basically I have a bunch of users and need to get their content from the database so I can search and page through it.
I'm using LINQ to SQL, my code at the moment it:
public IEnumerable<content> allcontent;
//Get users friends
IEnumerable<relationship> friends = from f in db.relationships
where f.userId == int.Parse(userId)
select f;
IEnumerable<relationship> freindData = friends.ToList();
foreach (relationship r in freindData)
{
IEnumerable<content> content = from c in db.contents
where c.userId == r.userId
orderby c.contentDate descending
select c;
// This is where I need to merge everything together
}
I hope that make some sense!
Matt
If I understand correctly what you are trying to do, why don't you try doing:
var result = from r in db.relationships
from c in db.contents
where r.userId == int.Parse(userId)
where c.userId == r.UserId
orderby c.contentDate descending
select new {
Relationship = r,
Content = c
}
This will give you an IEnumerable<T> where T is an anonymous type that has fields Relationship and Content.
If you know your users will have less than 2100 friends, you could send the keys from the data you already loaded back into the database easily:
List<int> friendIds = friendData
.Select(r => r.UserId)
.Distinct()
.ToList();
List<content> result = db.contents
.Where(c => friendIds.Contains(c.userId))
.ToList();
What happens here is that Linq translates each Id into a parameter and then builds an IN clause to do the filtering. 2100 is the maximum number of parameters that SQL server will accept... if you have more than 2100 friends, you'll have to break the ID list up and combine (Concat) the result lists.
Or, if you want a more literal answer to your question - Concat is a method that combines 2 IEnumerables together by creating a new IEnumerable which returns the items from the first and then the items from the second.
IEnumerable<content> results = Enumerable.Empty<content>();
foreach (relationship r in friendData)
{
IEnumerable<content> content = GetData(r);
results = results.Concat(content);
}
If you're doing an INNER join, look at the .Intersect() extension method.
Which things are you merging?
There are two main options you could use: .SelectMany(...) or .Concat(...)