So I've got my code trying to select an object from an array of objects, and if the object isn't found, I want to create my defaults.
lead_time = lead_times.select{|d| LeadTimeProfile.new unless d.day_of_week == day }
however, from what I can tell, this is not returning me the devault LeadTimeProfile.
is there a way of doing this? Or have I got it right?
So I've got my code trying to select an object from an array of objects, and if the object isn't found, I want to create my defaults.
Take a look at Enumerable#find
lead_time = lead_times.find{ |d| d.day_of_week == day } || LeadTimeProfile.new
filter your array first, and then do the construction
lead_time = lead_times.select{|d| d.day_of_week == day}.map {|d| LeadTimeProfile.new(d)}
Passing a lambda as a parameter also works.
lead_time = lead_times.find(lambda { LeadTimeProfile.new } ){ |d| d.day_of_week == day }
Here is another way to get the same results as what Kyle posted. There is no difference between this and using a or gate other than maybe making chaining method calls a bit cleaner.
day = 2
lead_times.find(-> { LeadTimeProfile.new }) { |p|
p.day_of_week == day
}.day_of_week
Related
I am trying to access a user object in a collection with the id = to users101 and set this to another users.
Controller.MyObject.SingleOrDefault(x => x.Id == "user101") = OtherUser();
Thanks in advance.
You can't do it with one LINQ expression.
Usually LINQ extensions works on enumerables, if MyObject is a collection you first have to find the required item and then overwrite it with the new object (moreover SingleOrDefault() will simply return null if condition is not satisfied).
You should write something like this (exact code depends on what MyObject is):
var item = Controller.MyObject.SingleOrDefault(x => x.Id == "user101");
if (item != null)
Controller.MyObject[Controller.MyObject.IndexOf(item)] = new OtherUser();
Please note that if you do not really need the check performed by SingleOrDefault() you can simplify the code (and avoid the double search performed in SingleOrDefault() and IndexOf()).
If this is "performance critical" maybe it is better to write an ad-hoc implementation that does this task in one single pass.
Try it in two lines:
var objectWithId = Controller.MyObject.SingleOrDefault(x => x.Id == "user101");
(objectWithId as WhateverTypeOfObjectOtherUserIs) = OtherUser();
I'm trying to figure out how to modify this to match on 2:
var result = _context.FirstOrDefault(c => c.CarId == carId);
I'm not sure how to tack this on. I just want to base it on c.CarId == carId && c.UserId == userId
where carId and userId are incoming params to my method that this LINQ statement resides in. I want to keep this as a lambda expression syntax.
Just do it exactly as you've written it:
var result = _context.FirstOrDefault(c => c.CarId == carId && c.UserId == userId);
There's nothing wrong with that. The lambda expression isn't restricted to compare a single property.
If you want to learn about LINQ in more detail, I'd start with LINQ to Objects, which is simpler to understand and predict. There are various tutorials around for it, and I have a blog series called Edulinq which examines each operator in detail.
I have an issue using the Dynamic Expression API. I cannot seem to compare a DataTable field against DBNull.Value. The API is supposed to be able to "support static field or static property access. Any public field or property can be accessed.". However given the following query:
var whatever = table1.AsEnumerable()
.Join(table2.AsEnumerable(),
(x) => x.Field<int>("Table1_ID"),
(y) => y.Field<int>("Table2_ID"),
(x, y) => new { x, y})
.AsQueryable()
.Where("x[\"NullableIntColumnName\"] == DBNull.Value");
I end up getting the error: "No property or field 'DBNull' exists in type '<>f__AnonymousType0`2'"
Anyone have ideas on how to get around this? I can't use Submission.Field("NullableIntColumnName") in the string passed to the Where method either, btw, or else I would be able to compare against null instead of DBNull.Value.
Well, I finally got it. cptScarlet almost had it.
var values = new object[] { DBNull.Value };
...
.Where("x[\"NullableIntColumnName\"] == #0", values);
or
.Where("x[\"NullableIntColumnName\"] == #0", DBNull.Value);
What happens when you replace your current .Where with something like
.Where(string.format("x[\"NullableIntColumnName\"] == {0}",DBNull.Value));
If you change x.Field<int>("Table1_ID") to x.Field<int?>("Table1_ID") then you'll get nullable integers instead of regular integers, and any DBNull values will be converted to simple C# null values. Based simply on your code snippet, I'm not even sure you'd need dynamic expressions - a simple .Where(foo => foo.x == null) ought to work.
In general, you can also try:
.Where("NullableColumnName.HasValue");
Sorry to non-answer with a USL but...
Have you looked in the source? There's not a lot of it. My guess is that DBNull is not in the list of registered root objects.
I dont have the source to hand right now, but it is also likely to tell you what any other constants one might compare against might be.
.Where(a => a.IntColName == null);
Edit:
Sorry, I did't see this dynamic requirement... Dynamic would be: (at least in Framework 4)
var intColName = "...";
.Where(string.Format("it.{0} is null", intColName));
Maybe a simple question, I'm trying to get a result from a table where the Name column contains all of an array of search terms. I'm creating a query and looping through my search strings, each time assigning the query = query.Where(...);. It appears that only the last term is being used, I supposed because I am attempting to restrict the same field each time. If I call .ToArray().AsQueryable() with each iteration I can get the cumlative restrinction behavior I'm looking for, but it there an easy way to do this using defered operators only?
Thanks!
If you're doing something like:
foreach (int foo in myFooArray)
{
query = query.where(x => x.foo == foo);
}
...then it will only use the last one since each where criteria will contain a reference to the 'foo' loop variable.
If this is what you're doing, change it to:
foreach (int foo in myFooArray)
{
int localFoo = foo;
query = query.where(x => x.foo == localFoo);
}
...and everything should be fine again.
If this is not what is happening, please provide a code sample of what you're doing...
I just asked this question. Which lead me to a new question :)
Up until this point, I have used the following pattern of selecting stuff with Linq to SQL, with the purpose of being able to handle 0 "rows" returned by the query:
var person = (from p in [DataContextObject].Persons
where p.PersonsID == 1
select new p).FirstOrDefault();
if (person == null)
{
// handle 0 "rows" returned.
}
But I can't use FirstOrDefault() when I do:
var person = from p in [DataContextObject].Persons
where p.PersonsID == 1
select new { p.PersonsID, p.PersonsAdress, p.PersonsZipcode };
// Under the hood, this pattern generates a query which selects specific
// columns which will be faster than selecting all columns as the above
// snippet of code does. This results in a performance-boost on large tables.
How do I check for 0 "rows" returned by the query, using the second pattern?
UPDATE:
I think my build fails because I am trying to assign the result of the query to a variable (this._user) declared with the type of [DataContext].User.
this._user = (from u in [DataContextObject].Users
where u.UsersID == [Int32]
select new { u.UsersID }).FirstOrDefault();
Compilation error: Cannot implicitly convert type "AnonymousType#1" to "[DataContext].User".
Any thoughts on how I can get around this? Would I have to make my own object?
Why can you keep doing the samething? Is it giving you an error?
var person = (from p in [DataContextObject].Persons
where p.PersonsID == 1
select new { p.PersonsID, p.PersonsAdress, p.PersonsZipcode }).FirstOrDefault();
if (person == null) {
// handle 0 "rows" returned.
}
It is still a reference object just like you actual object, it is just anonymous so you don't know the actual type before the code is compiled.
Update:
I see now what you were actually asking! Sorry, my answer no longer applies. I thought you were not getting a null value when it was empty. The accepted response is correct, if you want to use the object out of scope, you need to create a new type and just use New MyType(...). I know DevEx's RefactorPro has a refactoring for this, and I think resharper does as well.
Call .FirstOrDefault(null) like this:
string[] names = { "jim", "jane", "joe", "john", "jeremy", "jebus" };
var person = (
from p in names where p.StartsWith("notpresent") select
new { Name=p, FirstLetter=p.Substring(0,1) }
)
.DefaultIfEmpty(null)
.FirstOrDefault();
MessageBox.Show(person==null?"person was null":person.Name + "/" + person.FirstLetter);
That does the trick for me.
Regarding your UPDATE: you have to either create your own type, change this._user to be int, or select the whole object, not only specific columns.
if (person.Any()) /* ... */;
OR
if (person.Count() == 0) /* ... */;
You can still use FirstOrDefault. Just have
var PersonFields = (...).FirstOrDefault()
PersonFields will be be null or an object with those properties you created.