How to seed mysql database by running sql scripts in Ruby Rails platform?

I personally don't like using migrations to seed large amounts of "static" data. You can easily execute sql statements from db/seeds.rb by:

# Empty the soon-to-be-seeded table
MyModel.delete_all

# Get the current DB connection
connection = ActiveRecord::Base.connection();

# Execute a sql statement
connection.execute("Insert INTO my_models (id, name) VALUES
    (1, 'Example 1'),
    (2, 'Example 2'),
    (3, 'Example 3');")

Basically, once you have the current connection, you can just call a sql statement useing connection.execute("SQL CODE HERE").

If you generated your rails application from console, you should have db/seeds.rb file. It populates database tables with default values.

Then, you can execute rake db:seed.

The autogenerated seeds.rb contains this example, which should clear any doubts

# This file should contain all the record creation needed to seed the database with its default values.
# The data can then be loaded with the rake db:seed (or created alongside the db with db:setup).
#
# Examples:
#   
#   cities = City.create([{ :name => 'Chicago' }, { :name => 'Copenhagen' }])
#   Major.create(:name => 'Daley', :city => cities.first)

Note, that you do not put any sql statements in it (not usually). Although you can still execute sql from the seed file or migration, using ruby code and models is preferred.

on the comment
Maybe we have different understanding of what 'seed' means.
Anyway, migrations should help here.
It's too large topic to cover it in my post, so please read the guide. Although it focuses on creating/deleting tables, you can manipulate data just as well in a migration.

For initial, static daata use the seeds file as suggested.

For other purposes you can just wrap the entire insert statement inside an active record statement, e.g. find_by_sql("insert into mytable (col1, col2) values (1,2)"). using the "find by" isn't great reading but it'll do it.
You can also just use ActiveRecord::Base.connection.execute(sql) and put your sql there. Whatever you have such as insert, etc. will get executed. Usually you'll need a ; at the end if yu have multiple statements.