Linq. Aggregate

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  • shapper
    #1

    Linq. Aggregate

    Hello,

    I have the following Linq query:

    var q = (from p in database.Posts
    join pt in database.PostsT ags on p.PostID equals
    pt.PostID
    join t in database.Tags on pt.TagID equals t.TagID
    group t by p into pt
    select new PostPaper {
    Post = pt.Key,
    Tags = pt.ToList(),
    TagsCSV = pt.Aggregate((a , b) =a.Name + ", " +
    b.Name)
    }).ToPagedList( page.HasValue ? page.Value - 1 : 0,
    ListPageSize);

    I am getting an error on the aggregate function:
    Cannot implicitly convert type 'string' to 'BonsAlunos.Mod els.Tag'

    Basically, what I am trying to do is build TagsCSV with the Name
    property of all tags from Tags list separated by commas ... any idea
    of how to do this?

    Thanks,
    Miguel
    Tags: None
    • Jeroen Mostert
      #2
      Re: Linq. Aggregate

      shapper wrote:
      I have the following Linq query:
      >
      var q = (from p in database.Posts
      join pt in database.PostsT ags on p.PostID equals
      pt.PostID
      join t in database.Tags on pt.TagID equals t.TagID
      group t by p into pt
      select new PostPaper {
      Post = pt.Key,
      Tags = pt.ToList(),
      TagsCSV = pt.Aggregate((a , b) =a.Name + ", " +
      b.Name)
      }).ToPagedList( page.HasValue ? page.Value - 1 : 0,
      ListPageSize);
      >
      I am getting an error on the aggregate function:
      Cannot implicitly convert type 'string' to 'BonsAlunos.Mod els.Tag'
      >
      While you can use .Aggregate() for this, it's more trouble than it's worth
      (removing the final, unnecessary comma can't be done with an inline
      expression). Just use string.Join():

      TagsCSV = string.Join(", ", pt.Select(t =t.Name).ToArra y())

      And I'm getting a strong sense of deja vu here, because you used this exact
      expression in a post made not 24 hours ago in this newsgroup. It was good
      then, it's still good now. :-)

      --
      J.

        Comment

        • shapper
          #3
          Re: Linq. Aggregate

          On Sep 29, 12:18 am, Jeroen Mostert <jmost...@xs4al l.nlwrote:
          shapper wrote:
          I have the following Linq query:
          >
                var q = (from p in database.Posts
                            join pt in database.PostsT ags on p.PostID equals
          pt.PostID
                            join t in database.Tags on pt.TagIDequals t.TagID
                            group t by p into pt
                            select new PostPaper {
                              Post = pt.Key,
                              Tags = pt.ToList(),
                              TagsCSV = pt.Aggregate((a , b)=a.Name + ", " +
          b.Name)
                            }).ToPagedList( page.HasValue ? page..Value - 1 : 0,
          ListPageSize);
          >
          I am getting an error on the aggregate function:
          Cannot implicitly convert type 'string' to 'BonsAlunos.Mod els.Tag'
          >
          While you can use .Aggregate() for this, it's more trouble than it's worth
          (removing the final, unnecessary comma can't be done with an inline
          expression). Just use string.Join():
          >
             TagsCSV = string.Join(", ", pt.Select(t =t.Name).ToArra y())
          >
          And I'm getting a strong sense of deja vu here, because you used this exact
          expression in a post made not 24 hours ago in this newsgroup. It was good
          then, it's still good now. :-)
          >
          --
          J.
          Yes, you are right!

          But I saw something like this using Aggregate so I though it was a
          better way to do this.

          So I will keep String.Join.

          Thanks,
          Miguel

            Comment

            • Jeroen Mostert
              #4
              Re: Linq. Aggregate

              shapper wrote:
              On Sep 29, 12:18 am, Jeroen Mostert <jmost...@xs4al l.nlwrote:
              >shapper wrote:
              >>I have the following Linq query:
              >> var q = (from p in database.Posts
              >> join pt in database.PostsT ags on p.PostID equals
              >>pt.PostID
              >> join t in database.Tags on pt.TagID equals t.TagID
              >> group t by p into pt
              >> select new PostPaper {
              >> Post = pt.Key,
              >> Tags = pt.ToList(),
              >> TagsCSV = pt.Aggregate((a , b) =a.Name + ", " +
              >>b.Name)
              >> }).ToPagedList( page.HasValue ? page.Value - 1 : 0,
              >>ListPageSize) ;
              >>I am getting an error on the aggregate function:
              >>Cannot implicitly convert type 'string' to 'BonsAlunos.Mod els.Tag'
              >While you can use .Aggregate() for this, it's more trouble than it's worth
              >(removing the final, unnecessary comma can't be done with an inline
              >expression). Just use string.Join():
              >>
              > TagsCSV = string.Join(", ", pt.Select(t =t.Name).ToArra y())
              >>
              >And I'm getting a strong sense of deja vu here, because you used this exact
              >expression in a post made not 24 hours ago in this newsgroup. It was good
              >then, it's still good now. :-)
              >>
              >
              Yes, you are right!
              >
              But I saw something like this using Aggregate so I though it was a
              better way to do this.
              >
              Actually, it depends. If you expect that there will be a great number of
              PostPaper objects and .TagsCSV will only be evaluated sporadically, then
              using .Aggregate() may win out as its execution is deferred (unlike
              String.Join). This would be pretty unusual, though.

              On the other hand, if you expect .TagsCSV to be evaluated for every item,
              then .Aggregate() is certainly not better here. It essentially amounts to
              concatenating strings in a loop, which is what we're always told not to do
              (as it produces a lot of string garbage). Although creating the array with
              names has overhead too, it still scales better.

              Using an explicit StringBuilder is more efficient still, but that's hardly
              worth it.

              --
              J.

                Comment

                • shapper
                  #5
                  Re: Linq. Aggregate

                  On Sep 29, 12:57 am, Jeroen Mostert <jmost...@xs4al l.nlwrote:
                  shapper wrote:
                  On Sep 29, 12:18 am, Jeroen Mostert <jmost...@xs4al l.nlwrote:
                  shapper wrote:
                  >I have the following Linq query:
                  >      var q = (from p in database.Posts
                  >                  join pt in database.PostsT ags on p.PostID equals
                  >pt.PostID
                  >                  join t in database.Tags on pt.TagID equals t.TagID
                  >                  group t by p into pt
                  >                  select new PostPaper {
                  >                    Post = pt.Key,
                  >                    Tags = pt.ToList(),
                  >                    TagsCSV = pt.Aggregate((a , b) =a.Name + ", " +
                  >b.Name)
                  >                  }).ToPagedList( page.HasValue ? page.Value - 1 : 0,
                  >ListPageSize );
                  >I am getting an error on the aggregate function:
                  >Cannot implicitly convert type 'string' to 'BonsAlunos.Mod els.Tag'
                  While you can use .Aggregate() for this, it's more trouble than it's worth
                  (removing the final, unnecessary comma can't be done with an inline
                  expression). Just use string.Join():
                  >
                     TagsCSV = string.Join(", ", pt.Select(t =t.Name).ToArra y())
                  >
                  And I'm getting a strong sense of deja vu here, because you used this exact
                  expression in a post made not 24 hours ago in this newsgroup. It was good
                  then, it's still good now. :-)
                  >
                  Yes, you are right!
                  >
                  But I saw something like this using Aggregate so I though it was a
                  better way to do this.
                  >
                  Actually, it depends. If you expect that there will be a great number of
                  PostPaper objects and .TagsCSV will only be evaluated sporadically, then
                  using .Aggregate() may win out as its execution is deferred (unlike
                  String.Join). This would be pretty unusual, though.
                  >
                  On the other hand, if you expect .TagsCSV to be evaluated for every item,
                  then .Aggregate() is certainly not better here. It essentially amounts to
                  concatenating strings in a loop, which is what we're always told not to do
                  (as it produces a lot of string garbage). Although creating the array with
                  names has overhead too, it still scales better.
                  >
                  Using an explicit StringBuilder is more efficient still, but that's hardly
                  worth it.
                  >
                  --
                  J.
                  From your description String.Join seems the best solution.

                  Thank You,
                  Miguel

                    Comment

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