How to establish connection from a rake task? - mysql

Question

I need to create a rake task that gets the table rows from a MySQL database, parse the data, and insert into an Oracle database. The databases are on two different hosts.
My current attempt:
namespace :import_from_mysql do
class MySQLConnection < ActiveRecord::Base
self.abstract_class = true
establish_connection({
:adapter => 'mysql',
:host => 'xxx.xxx.com',
:database => 'sample_database',
:username => 'username',
:password => 'password'
})
end
class MySQLTable < MySQLConnection
self.table_name = "users"
self.primary_key = "id"
self.inheritance_column = "something_unique_here"
end
desc "Parse data before inserting to oracle database"
task :insert_to_oracle => :environment do |t|
puts "Rake task has begun...\n\n"
puts "Parsing data from MYSQL...\n\n"
MySQLTable.establish_connection
puts "Rake task has completed!"
end
end
But the MySQLTable.establish_connection establishes a connection to my local database which is sqlite even though I'm trying to connect to a mysql_adapter.
When I tried to establish a connection using the command below, I was able to connect to a MySQL adapter but I don't know how I can access my tables after the connection was established:
ActiveRecord::Base.establish_connection({:adapter => "mysql", :database => "sample_database", :host => "xxx.xxx.com", :username => "username", :password => "password" })
Any idea on why it keeps on connecting to sqlite? And after successfully establishing a connection to mysql, how do I select table rows after the MySQLTable.establish_connection statement?

Answer 1

With the connection generated using ActiveRecord::Base you can execute SQL statements against whatever database you connect to. Like so:
connection = ActiveRecord::Base.establish_connection(
:adapter => "mysql",
:host => "faraway",
:username => "myuser",
:password => "mypass",
:database => "somedatabase"
)
connection.execute('SELECT * FROM users')
Once established, the connection can also be referenced from ActiveRecord::Base class.
ActiveRecord::Base.establish_connection(...)
ActiveRecord::Base.connection.execute(...)

Answer 2

You can use mysql2 gem (https://github.com/brianmario/mysql2) inside your rake task.
client = Mysql2::Client.new(:host => "localhost", :username => "username", :database => "sample_database", :password => "password")
users = client.query("SELECT * FROM users")

Answer 3

Thought it might be helpful for someone else. The following worked for me!
connection = ActiveRecord::Base.establish_connection(
:adapter => "mysql",
:host => "faraway",
:username => "myuser",
:password => "mypass",
:database => "somedatabase"
)
#connection = ActiveRecord::Base.connection
result = #connection.exec_query('SELECT * FROM users')
result.each do |row|
puts row
end
Here, users is an already existing table in the "somedatabase".

Answer 4

although I am a noob with connections and "database back office" my trivial approach works like a charm, and there is no need with parameters in rake task that is for updating "domain tables" that contain only defining data like legal or help texts:
pre_db = ActiveRecord::Base.establish_connection('preproduction').connection
prod_db = ActiveRecord::Base.establish_connection('online').connection
and in database.yml I defined the databases. ("preproduction" is a step between dev and prod in my case and a real environment like "development". where 'online' is almost the same as 'production' (I made a different entry only for security reasons - not to destroy production if database.yml gets uploaded)