6 fundamental migration problems for beginners

Rails
1

Hi

I have 6 simple and fundamental Problem in RoR migration and although I
googled it and read many article about it and research it for a week , I
didn’t understand it. please help me for these problems :

1 - When I create a model (for example “rails generate model
Football_League”) , I see in db/migration a “create_football_leagues.rb”
. I want know what is the rules of RoR for changing the name that I gave
to my migration ? ( My migration’s name was FootballLeague and ruby
cretaed “create_football_leagues.rb”)

2 - What is the rules of naming ruby for creating a class in that file ?
( Ruby created a “CreateFootballLeague” class in
“create_football_leagues.rb”)

3 - After I use rake db:migrate , although I had 3 migration, I just see
such a image in cmd : ( I attached this image )
and I didn’t any migration for another 3 migrations whereas sometimes
ago when I run a migration with rake db:migrate, all my migrations are
migrated.

4 - After I type rake db:migrate and after that Rake execute
“create_football_leagues.rb” I go to mysql and I saw a table with
“schema_migrations” name. and in that table there are some versions of
my migrations. now I want to know what is the application of such a
table ?

5 - In http://guides.rubyonrails.org I read that if we want to change
the primary key of a table from default id to our own field we must use
:primary_key option but I didn’t understand how can I use it .If someone
can give an example for this subject.

6 - What is the difference between this 2 commands : 1 - “rails generate
migration footabll” 2 - rails generate model football. I knew that
with “rails generate model football” we must some extra file in
app/model but I don’t understand function of those files.

My friend please help me to solve these fundamental problems . special
thanks .

2

On Tue, Jan 10, 2012 at 5:41 PM, amir e. [email protected] wrote:

to my migration ? ( My migration’s name was FootballLeague and ruby

migration footabll" 2 - rails generate model football. I knew that

Study “Rails Guides” http://guides.rubyonrails.org/

  • Getting started
  • Migrations
  • check for “generators”
  • …

HTH,

Peter

3

On 10 January 2012 16:41, amir e. [email protected] wrote:

Hi

I have 6 simple and fundamental Problem in RoR migration and although I
googled it and read many article about it and research it for a week , I
didn’t understand it. please help me for these problems :

Rather than answer the questions in detail I suggest that you work
right through a good tutorial such as railstutorial.org, which is free
to use online, then many such questions will be answered. Also look
at the Rails Guides.
Make sure that you have installed the version of rails that matches
the tutorial you are using.

I will say, though, do not change the primary key from id unless you
absolutely must do this, to cope with a legacy database. Otherwise
you will make life difficult.

Colin

3 - After I use rake db:migrate , although I had 3 migration, I just see
“create_football_leagues.rb” I go to mysql and I saw a table with
:primary_key option but I didn’t understand how can I use it .If someone

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4

Study “Rails Guides” http://guides.rubyonrails.org/

  • Getting started
  • Migrations
  • check for “generators”
  • …

HTH,

Peter

I study it , and if I didn’t understand.

5

On 01/10/2012 09:41 AM, amir e. wrote:

to my migration ? ( My migration’s name was FootballLeague and ruby
cretaed “create_football_leagues.rb”)

The name of the migration is only of passing interest. What it is named
has very little to do with what it does. The whole idea of the
migrations is to create/delete database tables and indexes and to
add/remove columns to existing tables and sometimes to insert small
amounts of initial data into tables (not recommended). The important
thing in the migrations is the names of the tables and the names of the
columns. The name of the migration is just so you recall what that
migration was all about.

2 - What is the rules of naming ruby for creating a class in that file ?
( Ruby created a “CreateFootballLeague” class in
“create_football_leagues.rb”)

As in the migration name, the name of the class that is in that file is
of little interest. It is only used when you run a migration and really
has nothing to do with what the migration really does. The class is
bound to the name of the file by some rules that are not really very
interesting. Just go with it and don’t worry about it.

4 - After I type rake db:migrate and after that Rake execute
“create_football_leagues.rb” I go to mysql and I saw a table with
“schema_migrations” name. and in that table there are some versions of
my migrations. now I want to know what is the application of such a
table ?

The schema_migrations table is used to keep track of which migrations
have been run. In 3 above all of the previously defined migrations have
run so the only one that will be run is the new one. If you change a
migration file which has already been run it will not be rerun. Only
the migrations on a whole are tracked. If you want to run all of the
migrations you can just ‘rake db:drop db:create db:migrate’. Of course
you will loose any data that is in the database.

5 - In http://guides.rubyonrails.org I read that if we want to change
the primary key of a table from default id to our own field we must use
:primary_key option but I didn’t understand how can I use it .If someone
can give an example for this subject.

Don’t do it. Rails works best and most easily if you go with the flow.

6 - What is the difference between this 2 commands : 1 - “rails generate
migration footabll” 2 - rails generate model football. I knew that
with “rails generate model football” we must some extra file in
app/model but I don’t understand function of those files.

You need to read the tutorials on models. The migration generates the
database tables. The model files define relations among the different
Active Record classes that are formed from those columns. At a minimum
a model file is required for each table (that you want to use with
Active Record). The file will look like:class Column < ActiveRecord::Base end<The relations are descirbed in the model files.

Norm

6

On Tue, Jan 10, 2012 at 6:21 PM, amir e. [email protected] wrote:

I study it , and if I didn’t understand.

OK, great.

A good way to get better responses on a mailing is to
mention e.g. “what did you read”, “what did you try”,
“what was the outcome”, “why do you think that is not
correct” etc.

E.g. from this guide:

http://catb.org/~esr/faqs/smart-questions.html

"
Before You Ask

Before asking a technical question by e-mail, or in a newsgroup, or on a
website chat board, do the following:

Try to find an answer by searching the archives of the forum you plan to
post to.

Try to find an answer by searching the Web.

Try to find an answer by reading the manual.

Try to find an answer by reading a FAQ.

Try to find an answer by inspection or experimentation.

Try to find an answer by asking a skilled friend.

If you’re a programmer, try to find an answer by reading the source
code.

When you ask your question, display the fact that you have done these
things first; this will help establish that you’re not being a lazy
sponge
and wasting people’s time. Better yet, display what you have learned
from
doing these things. We like answering questions for people who have
demonstrated they can learn from the answers.
"

Hope This helps and welcome :slight_smile:

Peter

–
Peter V.
http://twitter.com/peter_v
http://rails.vandenabeele.com