Linq replace column value with another value

Question

Well, im doing a linq query to get a list of results with the same column, and then i need to replace that column value with a new one.

First Code:

var db = GetContext();
var result = from f in GetContext().ProjectStateHistories
             where f.ProjectId.Equals(oldProjectId)
             select f;

foreach (var item in result)
{
     var projectStateHistoryUpdate = db.ProjectStateHistories.Find(item.Id);
     projectStateHistoryUpdate.ProjectId = newProjectId;
     db.Entry(projectStateHistoryUpdate).State = EntityState.Modified;
}
db.SaveChanges();

I searched for some answers, and i found that i can use Select, and make a new object (Linq replace null/empty value with another value)

Second Code:

var result = (from f in GetContext().ProjectStateHistories
              where f.ProjectId.Equals(oldProjectId)
              select f).Select(d=> new { Id = d.Id, EventName = d.EventName, LogUser = d.LogUser, ProjectId = newProjectId, TimeStamp = d.TimeStamp });

And even, Third Code:

var db = GetContext();
var result = (from f in db.ProjectStateHistories
              where f.ProjectId.Equals(oldProjectId)
              select f).Select(d=> new { ProjectId = newProjectId});

But only the First Code works.

I wanted to ask what i am doing wrong, since i think it is better to change the value with a query, instead of using a foreach.

2nd and 3rd, you are reading the data, not updating it back ! — Shyju
– Shyju, Commented May 6, 2016 at 14:39
What's wrong with the first code fragment? Why do yo want to do it differently? — Gert Arnold
– Gert Arnold, Commented May 6, 2016 at 14:41
1. do you really need to SaveChanges in the foreach loop?.2. Why are you calling GetContext() again in the query instead of using db? — raven
– raven, Commented May 6, 2016 at 14:41
@GertArnold first code is working great, but i wanted to avoid the foreach. — pierregomes
– pierregomes, Commented May 6, 2016 at 14:41
@RobertoDeLaParra yah, savechanges must be out of the foreach, your are right thanks. About the GetContext, i m using db, my fault sorry. — pierregomes
– pierregomes, Commented May 6, 2016 at 14:47

Riad Baghbanli · Accepted Answer · 2016-05-06 14:59:07Z

5

See code below:

var db = GetContext();
(from f in db.ProjectStateHistories
     where f.ProjectId.Equals(oldProjectId)
     select f)
       .ToList()
       .ForEach(i => i.ProjectId = newProjectId);
db.SaveChanges();

Alternatively:

var db = GetContext();
db.ProjectStateHistories
       .Where(f => f.ProjectId.Equals(oldProjectId))
       .ToList()
       .ForEach(f => f.ProjectId = newProjectId);
db.SaveChanges();

edited May 6, 2016 at 14:59

answered May 6, 2016 at 14:56

Riad Baghbanli

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3 Comments

pierregomes Over a year ago

Thanks a lot @Riad ! First Code worked, what do you think is better to use ? The first one, or the second one ?

Phillip Morton Over a year ago

@pierregomes This too has a for each loop in it ;) Its just refactored ;)

Riad Baghbanli Over a year ago

I prefer the method notation (the second one). I find it easier to read than query notation.

Community · Accepted Answer · 2017-05-23 11:53:52Z

2

The shortest way I know of to replace your code is this:

var db = getcontext();

db.ProjectStateHistories
    .Where(f => f.ProjectId.Equals(oldProjectId))
    .ToList()
    .ForEach(f => f.ProjectId = newProjectId);

db.SaveChanges();

1 Comment

pierregomes Over a year ago

Sorry for not answering. This works too :) Thanks a lot for your time!

Phillip Morton · Accepted Answer · 2016-05-06 15:04:01Z

0

I've just had a thought that could help you, I am just free coding here!

If you just put the for each as part of the select, and then save your changes will that work?

foreach (var source in db.ProjectStateHistories.Where(x => x.ProjectId == oldProjectId)) 
{ 
     source.ProjectId= newProjectId; 
     db.Entry(source).State = EntityState.Modified; 
} 
db.SaveChanges();

I think this is a more efficient way of doing it.

Also the .Select() method is only really useful if you need to Project to a view Model, it won't change the variables in the database, just show them in the newly declared object.

Thanks,

Phill

answered May 6, 2016 at 15:04

Phillip Morton

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2 Comments

pierregomes Over a year ago

Thanks a lot @Phillip :) Then there is no change between your code and Riad's code ? Because yours it's easier to read. That's a nice solution too. I made this question because i thought i should avoid foreach

pierregomes Over a year ago

I am using your code, because it is easier to read :) Thanks a lot for your time.

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