Preserving has_many order

My application will need to store many measurements (eg: temperature)
for a set of locations (eg: roof) and periodically recall those
measurements for charting, browsing, or editing.

class Location < ActiveRecord::Base
has_many :Measurements
end

class Measurement < ActiveRecord::Base
belongs_to :Location
end

I would like the measurements to be kept in the order in which they
are taken and stored in the database so that a potentially expensive
sorting operation is not required every time the user queries a
location’s temperature history.

If I implemented this with a linked list, I would add new temperature
measurements to the head and traverse down the list as needed for a
deterministic retrieval. Will Rails perform a conceptually similar
operation and store/retrieve Measurement instances in a determinsitic
manner? If not, are there any recommendations for preserving order in
a has_many relationship?

Thanks!
Mark

Frederick C. wrote:

On Feb 2, 5:26�am, MarkB [email protected] wrote:

a has_many relationship?

You should add an order clause to your has_many, without one the
database is free to return rows in any order it pleases. This
shouldn’t be a performance issue as long as you have appropriate
indexes in place.

There is a plugin that simplifies the implementation of what is
described in the OP. I have no idea whether this plugin is still a
viable solution. There may be newer ones that replace its functionality.
But, I think it’s still worth taking a look at for ideas:

http://github.com/rails/acts_as_list/

On Feb 2, 5:26 am, MarkB [email protected] wrote:

a has_many relationship?

You should add an order clause to your has_many, without one the
database is free to return rows in any order it pleases. This
shouldn’t be a performance issue as long as you have appropriate
indexes in place.

Fred

Robert W. wrote:

Frederick C. wrote:

On Feb 2, 5:26�am, MarkB [email protected] wrote:

a has_many relationship?

You should add an order clause to your has_many, without one the
database is free to return rows in any order it pleases. This
shouldn’t be a performance issue as long as you have appropriate
indexes in place.

There is a plugin that simplifies the implementation of what is
described in the OP. I have no idea whether this plugin is still a
viable solution. There may be newer ones that replace its functionality.
But, I think it’s still worth taking a look at for ideas:

http://github.com/rails/acts_as_list/

That’s what I’d use. It’s simple and seems to work well.

Best,
–Â
Marnen Laibow-Koser
http://www.marnen.org
[email protected]

MarkB wrote:

Thanks everyone for the recommendations. :order looks like an easy
way to get started. I’ll go with that and see if any optimizations
are needed down the road.

On Feb 2, 12:37�am, Frederick C. [email protected]

The question is, “what to order by?” You can’t truly depend on ordering
by an auto-generated primary key. That might work in some databases, but
certainly not for all. Even with databases that do seem to keep records
in order by a simple incrementing integer primary key, there is no
guarantee.

You might think then that ordering by the created_at date could be an
option. However, that’s risky too. Ordering could easily be corrupted by
changes to the system clock.

Something like acts_as_list maintains a separate :position column in the
database that can be used to both ensure proper ordering as records are
created, and allows for reordering of the list when (or if) necessary.

Thanks everyone for the recommendations. :order looks like an easy
way to get started. I’ll go with that and see if any optimizations
are needed down the road.

On Feb 2, 12:37 am, Frederick C. [email protected]

Robert W. wrote:

Robert W. wrote:

Something like acts_as_list maintains a separate :position column in the
database that can be used to both ensure proper ordering as records are
created, and allows for reordering of the list when (or if) necessary.

Come to think of it, this brings up an interesting question. Does any
know if acts_as_list is subject to race conditions when
assigning/maintaining the position attribute?

I doubt it. All AR saves and updates are transactional.

Best,
–Â
Marnen Laibow-Koser
http://www.marnen.org
[email protected]

Milan D. wrote:

Even with databases that do seem to keep records
in order by a simple incrementing integer primary key, there is no
guarantee.

Can you explain this a little further?

Well, the counter could always get reset. But there’s an important
principle here: the value of a surrogate key should never be meaningful.
You should be able to replace all your primary keys with, say, GUIDs and
have your app keep working.

Best,
–Â
Marnen Laibow-Koser
http://www.marnen.org
[email protected]

Even with databases that do seem to keep records
in order by a simple incrementing integer primary key, there is no
guarantee.

Can you explain this a little further?

Robert W. wrote:

Something like acts_as_list maintains a separate :position column in the
database that can be used to both ensure proper ordering as records are
created, and allows for reordering of the list when (or if) necessary.

Come to think of it, this brings up an interesting question. Does any
know if acts_as_list is subject to race conditions when
assigning/maintaining the position attribute?

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