Re: [DataMapper] get previous/next item in database

[Ben Lovell]

On 22 January 2013 10:44, Alex a...@sicanstudios.com wrote:

 Hi,
 I’m trying to get the previous and next item (articles) from a database
 with Datamapper. I thought of doing it with the serial identifier

 property :id, Serial


 and increment/decrease this number to find the previous/next posts of that
 particular article, but it doesn’t work. Because the serial is incremented
 fine for new articles (1,2,3,4,5), but if you delete say id: #3, then the
 next article of #2 will be #4, not #3.

 Do you know if there is an easier/more correct way to do it with
 Datamapper?


Hey Alex,

Sounds like you're missing a concept in your application. Perhaps you
should add some kind of ordering/page column to your table if you need to
be explicit about ordering? This way you could rearrange the ordering if
necessary, ensure uniqueness for pages/ordering and other goodness.

Alternatively, if you're storing created_at timestamps on your tables you
could use this as a surrogate for ordering.

Cheers,
Ben

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Re: [DataMapper] get previous/next item in database

[kristian]

what about doing something like this :

current_id = Audit.first.id
next = Audit.first( :id.gt = current_id )

- Kristian

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Re: [DataMapper] get previous/next item in database

[Alex]

Thanks Ben. I do store timestamps in the table. Would you suggest I should 
check for older/newer timestamps inside this loop?

  #defines if a post of a user exists in db
   def post_in_db
 posts_of_user.each do |post|
   if post.slug == params[:post_slug]
 @post_slug ||= post.slug
 @post_title ||= post.title
 @post_body ||= post.body
 @post_date ||= post.created_on
   end
 end
 if @post_body == nil 
   redirect ('/' + params[:nickname])
 end
   end


or I should store the @post_date and create a new helper with a new loop to 
find the immediate older/newer timestamp before/after the @post_date? I’m 
thinking in terms of performance the first method would be better.

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Re: [DataMapper] get previous/next item in database

[Alex]

@Kristian Thanks, I haven’t tried it yet, but how does that get the next 
id? With the :id.gt ? How about the previous id?

On Tuesday, January 22, 2013 1:13:51 PM UTC+2, kristian wrote:

 what about doing something like this : 

 current_id = Audit.first.id 
 next = Audit.first( :id.gt = current_id ) 

 - Kristian 


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Re: [DataMapper] get previous/next item in database

[Ben Lovell]

On 22 January 2013 12:04, Alex a...@sicanstudios.com wrote:

 Thanks Ben. I do store timestamps in the table. Would you suggest I should
 check for older/newer timestamps inside this loop?

   #defines if a post of a user exists in db
   def post_in_db
 posts_of_user.each do |post|
   if post.slug == params[:post_slug]
 @post_slug ||= post.slug
 @post_title ||= post.title
 @post_body ||= post.body
 @post_date ||= post.created_on
   end
 end
 if @post_body == nil
   redirect ('/' + params[:nickname])
 end
   end


 or I should store the @post_date and create a new helper with a new loop
 to find the immediate older/newer timestamp before/after the @post_date?
 I’m thinking in terms of performance the first method would be better.

 I'd just do a check for created_on  current for previous and created_on 
current for next and just call `first` with the predicate.

For example:

class Post
  def prev
first(:created_at.lt = self.created_at)
  end

  def next
first(:created_at.gt = self.created_at)
  end

  #...
end

Note: This is gmail code and unchecked/tested :)

Cheers,
Ben

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Re: [DataMapper] get previous/next item in database

[kristian]

gt == great-than

so the previous id you get with

previous = Audit.first( :id.lt = current_id )

lt == less-than or lower-than or so

and id are unique per definition and they get automatically created by
sequence or similar

FYI next and previous can be nil !

- Kristian
On Tue, Jan 22, 2013 at 5:36 PM, Alex a...@sicanstudios.com wrote:
 @Kristian Thanks, I haven’t tried it yet, but how does that get the next id?
 With the :id.gt ? How about the previous id?


 On Tuesday, January 22, 2013 1:13:51 PM UTC+2, kristian wrote:

 what about doing something like this :

 current_id = Audit.first.id
 next = Audit.first( :id.gt = current_id )

 - Kristian

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Re: [DataMapper] get previous/next item in database

[Alex]

Thanks guys, the code works, however there is a problem. The .gt works 
fine, but the .lt always gives me the first element in the db, not the 
previous element! I tried it with both the :id (@post_id) and the 
:created_on (@post_date) properties, getting the same result:

  def post_in_db
 posts_of_user.each do |post|
   if post.slug == params[:post_slug]
 @post_slug ||= post.slug
 @post_title ||= post.title
 @post_body ||= post.body
 @post_date ||= post.created_on
 @post_private ||= post.private
 @post_id ||= post.id
 @previous = prev.title
 @next = nexts.title
   end
 end
   end
   def prev
 Post.first(:created_on.lt = @post_date)
   end
   def nexts
 Post.first(:created_on.gt = @post_date)
   end


Any ideas?

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Re: [DataMapper] get previous/next item in database

[Ben Lovell]

Add some ordering to the posts, this should return the correct records:

On 22 January 2013 17:13, Alex a...@sicanstudios.com wrote:

 Thanks guys, the code works, however there is a problem. The .gt works
 fine, but the .lt always gives me the first element in the db, not the
 previous element! I tried it with both the :id (@post_id) and the
 :created_on (@post_date) properties, getting the same result:

   def post_in_db
 posts_of_user.each do |post|
   if post.slug == params[:post_slug]
 @post_slug ||= post.slug
 @post_title ||= post.title
 @post_body ||= post.body
 @post_date ||= post.created_on
 @post_private ||= post.private
 @post_id ||= post.id
 @previous = prev.title
 @next = nexts.title
   end
 end
   end
   def prev
 Post.first(:created_on.lt = @post_date, :order = [:post_date.asc])
   end
   def nexts
 Post.first(:created_on.gt = @post_date, :order = [:post_date.asc])
   end


 Any ideas?


Again, I have to add this is untested gmail code ;)

Cheers,
Ben

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Re: [DataMapper] get previous/next item in database

[Alex]

Thanks a lot, Ben! Got it working by ordering the one in descending and the 
other in ascending order:

  def prev
 Post.first(:created_on.lt = @post_date, :order = [:created_on.desc])
   end
   def nexts
 Post.first(:created_on.gt = @post_date, :order = [:created_on.asc])
   end


Thanks guys,
Alex 

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Re: [DataMapper] get previous/next item in database

[kristian]

take last instead of first :)

 Audit.last( :id.lt = 10 )
= #Audit @id=9

- Kristian

On Tue, Jan 22, 2013 at 10:43 PM, Alex a...@sicanstudios.com wrote:
 Thanks guys, the code works, however there is a problem. The .gt works fine,
 but the .lt always gives me the first element in the db, not the previous
 element! I tried it with both the :id (@post_id) and the :created_on
 (@post_date) properties, getting the same result:

   def post_in_db
 posts_of_user.each do |post|
   if post.slug == params[:post_slug]
 @post_slug ||= post.slug
 @post_title ||= post.title
 @post_body ||= post.body
 @post_date ||= post.created_on
 @post_private ||= post.private
 @post_id ||= post.id
 @previous = prev.title
 @next = nexts.title
   end
 end
   end
   def prev
 Post.first(:created_on.lt = @post_date)
   end
   def nexts
 Post.first(:created_on.gt = @post_date)
   end


 Any ideas?

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Re: [DataMapper] get previous/next item in database

[kristian]

be aware that timestamps are not unique, i.e. they usually have no
millis or nanos. just a thought whether using them to iterate over
them.

- Kristian

On Tue, Jan 22, 2013 at 11:16 PM, kristian m.krist...@web.de wrote:
 take last instead of first :)

 Audit.last( :id.lt = 10 )
 = #Audit @id=9

 - Kristian

 On Tue, Jan 22, 2013 at 10:43 PM, Alex a...@sicanstudios.com wrote:
 Thanks guys, the code works, however there is a problem. The .gt works fine,
 but the .lt always gives me the first element in the db, not the previous
 element! I tried it with both the :id (@post_id) and the :created_on
 (@post_date) properties, getting the same result:

   def post_in_db
 posts_of_user.each do |post|
   if post.slug == params[:post_slug]
 @post_slug ||= post.slug
 @post_title ||= post.title
 @post_body ||= post.body
 @post_date ||= post.created_on
 @post_private ||= post.private
 @post_id ||= post.id
 @previous = prev.title
 @next = nexts.title
   end
 end
   end
   def prev
 Post.first(:created_on.lt = @post_date)
   end
   def nexts
 Post.first(:created_on.gt = @post_date)
   end


 Any ideas?

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Re: [DataMapper] get previous/next item in database

[Ben Lovell]

On 22 January 2013 17:51, kristian m.krist...@web.de wrote:

 be aware that timestamps are not unique, i.e. they usually have no
 millis or nanos. just a thought whether using them to iterate over
 them.

 - Kristian

 On Tue, Jan 22, 2013 at 11:16 PM, kristian m.krist...@web.de wrote:
  take last instead of first :)
 
  Audit.last( :id.lt = 10 )
  = #Audit @id=9
 
  - Kristian
 


Heh, first time I've ever participated in pair-programming via email :)

Cheers,
Ben


  On Tue, Jan 22, 2013 at 10:43 PM, Alex a...@sicanstudios.com wrote:
  Thanks guys, the code works, however there is a problem. The .gt works
 fine,
  but the .lt always gives me the first element in the db, not the
 previous
  element! I tried it with both the :id (@post_id) and the :created_on
  (@post_date) properties, getting the same result:
 
def post_in_db
  posts_of_user.each do |post|
if post.slug == params[:post_slug]
  @post_slug ||= post.slug
  @post_title ||= post.title
  @post_body ||= post.body
  @post_date ||= post.created_on
  @post_private ||= post.private
  @post_id ||= post.id
  @previous = prev.title
  @next = nexts.title
end
  end
end
def prev
  Post.first(:created_on.lt = @post_date)
end
def nexts
  Post.first(:created_on.gt = @post_date)
end
 
 
  Any ideas?
 
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 Groups
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Re: [DataMapper] get previous/next item in database

[kristian]

is a pleasure :))

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Re: [DataMapper] get previous/next item in database

[Alex]

Yeah, it works with the .last. I might switch to the :id.

Thanks Kristian and Ben! You rock!
Alex



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