Why is AR migrations using one file per migration?

Hi, I’m currently in the process of rethinking some of the design
decisions
I’ve taken in my first attempt to add migrations support to Lobos, a
Clojure
schema manipulation library. I’ve based this work mostly on
ActiveRecord,
but now that I’m getting some feedback on it, I really wonder why Rails
migrations are spread around into individual files. Wouldn’t it be
better to
simply add the timestamp to the migrations name and put them all into
one
file?

Here’s the discussion that made me ask this question:

https://groups.google.com/d/topic/lobos-library/GnFBKahrhY0/discussion

Wouldn’t it be better to simply add the timestamp to the migrations name
and put them all into one file?

Not really. Consider this simple scenario for one example of why it’s a
bad
idea to have them in the same file.

I check out the source code to our application and add a migration.
You check out the source code to our application and add a different
migration.

If they are in the same file, we have a conflict when it’s time to check
in
our code.
If they are in separate files, there won’t be any conflict when we both
check in our code.

On May 14, 8:58pm, Nicolas B. [email protected] wrote:

Hi, I’m currently in the process of rethinking some of the design decisions
I’ve taken in my first attempt to add migrations support to Lobos, a Clojure
schema manipulation library. I’ve based this work mostly on ActiveRecord,
but now that I’m getting some feedback on it, I really wonder why Rails
migrations are spread around into individual files. Wouldn’t it be better to
simply add the timestamp to the migrations name and put them all into one
file?

Just off the top of my head, it is probably easier to write a
generator that creates a new migration file than one that edits an
existing file

Fred

Yes, that’s about the worse drawback I had thought about, but this issue
is
quite moot. One dev will only have to do a proper merge and that’s it.

On Sun, May 15, 2011 at 11:35 AM, Nicolas B.
[email protected]wrote:

Yes, that’s about the worse drawback I had thought about, but this issue is
quite moot. One dev will only have to do a proper merge and that’s it.

Right. It is actually a really good question you raise, and yes a merge
would not be any different than two devs who have modified any other
file,
git handles that gracefully. One thing I thoght of that at first I
thought
would make a one file situation more difficult but on reflection make a
one
file situation easier is rolling up migrations — once I accumulate a
lot
of migrations I roll them up and dump the contents of the schema into
the
initial migration. Having one file would allow me just to delete text
and
copy rather than having to delete files. Not a big deal but one less
step.

On Sunday, May 15, 2011 12:44:08 PM UTC-4, DK wrote:

Right. It is actually a really good question you raise, and yes a merge
would not be any different than two devs who have modified any other file,
git handles that gracefully. One thing I thoght of that at first I thought
would make a one file situation more difficult but on reflection make a one
file situation easier is rolling up migrations — once I accumulate a lot
of migrations I roll them up and dump the contents of the schema into the
initial migration. Having one file would allow me just to delete text and
copy rather than having to delete files. Not a big deal but one less step.

Yes, that would be a nice advantage. For Lobos, this could go one step
further, as Clojure evaluates source code linearly, I could make the
migrations ordered by their position into the file, thus making it
really
easy to reorder migrations.

On Sunday, May 15, 2011 1:05:30 PM UTC-4, DK wrote:

of migrations I roll them up and dump the contents of the schema into the
Hmm. but how would that work if you have dependencies? Like if you move a
migration which adds a field to a table before you create the table?

That would be the responsibility of the developer for now. I could make
my
library analyze the migrations to find such conflicts, that might be a
feature to add in some future release, but I’m not sure it’s worth it.

What would be cool is that you could order the file in any order by Rails
would run the migrations in sequence regardless of the order in the file (I
am assuming each ‘migration’ in the file would have some sort of id or
timestamp).

Unless I’m mistaken, that would be the only way to implement the ‘all
migrations into one file’ feature in Ruby, at least without resorting to
top-level calls ensuring the order of evaluation. Although I must say
that
I’m not that good at Ruby meta-programming.

Thinking more, one more advantage is that it would be easier to find
changes say for a specific table. Right now I just have to grep for it
unless I can read the five mile long name I have given the file.

Yeah, while thinking about this, I realized how tiring it can be to
search/consult/erase migrations files and I’m more and more sold to put
all
migrations into one file.

On Sun, May 15, 2011 at 11:53 AM, Nicolas B.
[email protected]wrote:

Yes, that would be a nice advantage. For Lobos, this could go one step
further, as Clojure evaluates source code linearly, I could make the
migrations ordered by their position into the file, thus making it really
easy to reorder migrations.

Hmm. but how would that work if you have dependencies? Like if you move
a
migration which adds a field to a table before you create the table?
What
would be cool is that you could order the file in any order by Rails
would
run the migrations in sequence regardless of the order in the file (I am
assuming each ‘migration’ in the file would have some sort of id or
timestamp). Thinking more, one more advantage is that it would be easier
to
find changes say for a specific table. Right now I just have to grep for
it
unless I can read the five mile long name I have given the file.

On 15 May 2011 18:22, Nicolas B. [email protected] wrote:

Yeah, while thinking about this, I realized how tiring it can be to
search/consult/erase migrations files and I’m more and more sold to put all
migrations into one file.

Why go half-way? Put all the models in one file too, bundle all the
controllers in with them, and you can whip the helpers in at the
bottom (we don’t need them often)

Alternatively, there’s no serious issue at all with having migrations
in separate files - yes, in one file I can merge them in my source
control software, but if they’re in their own files I don’t have to
bother. For the zero benefit that one migration file over lots of
files gives me, I can avoid the extra niggle of having to resolve
merge conflicts in it.

:-/

PS What do you ever search migrations for? Everything I’ve ever needed
is in schema.rb, or I just look in the DB…

On Sun, May 15, 2011 at 12:32 PM, Michael P. [email protected]
wrote:

hahaa, I get it but if you want to make the analogy exact, it would be
to
change rails so every time a model is changed it creates a new file that
extends the model. And then we would have the generator which puts
everything into one big file, like the schema :slight_smile:

On Sunday, May 15, 2011 1:32:26 PM UTC-4, pavling wrote:

Lol, good one, but more seriously, the problem with migrations is just
that
the timestamps comes first making annoying to open in Emacs. That’s a
very
personal issue though, as I never got used to things like ido-mode.

Alternatively, there’s no serious issue at all with having migrations
in separate files - yes, in one file I can merge them in my source
control software, but if they’re in their own files I don’t have to
bother. For the zero benefit that one migration file over lots of
files gives me, I can avoid the extra niggle of having to resolve
merge conflicts in it.

I understand and I’m not condemning AR on that design decision, which is
perfectly fine. I’m just trying to see if there’s some issue I’m
overlooking
by putting all migrations into one file, which could have more concrete
advantages in the library I’m writing.

PS What do you ever search migrations for? Everything I’ve ever needed
is in schema.rb, or I just look in the DB…

Searching migrations is not something I’m doing often, but it could help
sometimes to investigate why something have been done in a certain way.

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