How can I switch database inside a Rails console on Heroku? - ruby-on-rails

Question

I'm trying to migrate users from one system to another. Each system has its own database and different classes.
My plan is to connect to one database, read some info from one database via SQL commands:
ActiveRecord::Base.connection.execute(sql_command)
do something with the data and then write some results on the new database using normal models.
I plan on doing that inside Sidekiq job but I'm trying to do some testing using a Rails console on Heroku.
This should be pretty straightforward, but this proves ridiculously difficult.
When I launched a Rails console on Heroku. I'm connecting to DATABASE_URL, which is ok, but when I try to connect to the old database and execute a command, like this:
ActiveRecord::Base.establish_connection(adapter: "postgresql", encoding: "unicode", pool: 10, url: "postgres://...info...")
ActiveRecord::Base.connection.execute("select count(*) from users")
I end up with:
PG::ConnectionBad (could not connect to server: No such file or directory)
Is the server running locally and accepting
connections on Unix domain socket "/var/run/postgresql/.s.PGSQL.5432"?
I can, however, connect to this old database by launching my rails console on heroku using DATABASE_URL as the env variable:
$ heroku run 'DATABASE_URL=postgres://...info... rails console' -a console' -a app
BUT I don't know how to switch back to my new database so I can update things.
How does one switch databases at run time when using rails console on heroku?

Answer 1

Why try to runtime switch the database? Why not have both connected at the same time and specify at the model level which database they read/write from? Rails supports connecting multiple databases and specifying in individual models what database connection to use: https://guides.rubyonrails.org/active_record_multiple_databases.html

Answer 2

The problem was that using url: didn't work and all parameters needed to be specified.
config = {"adapter"=>"postgresql", "encoding"=>"unicode", "pool"=>10, "username"=>"u...", "password"=>"p...", "port"=>5432, "database"=>"d...", "host"=>"ec2..."}
If you go for a 3tier database yml, you can use this:
config = ActiveRecord::Base.configurations["production"]["seconddb"]
Then, you can use establish_connection
ActiveRecord::Base.establish_connection(config)
ActiveRecord::Base.connection.execute("select count(*) from users")
Once I started specifying username, password, port, database and host, it all worked like a charm.
To work with both databases at the same time, a good way is to create a class
class OtherDB < ActiveRecord::Base
establish_connection(ActiveRecord::Base.configurations["production"]["seconddb"])
end
Then you can call things that way
OtherDB.table_name = "table_name"
OtherDB.first
ref (Establish a connection to another database only in a block?)
And to run SQL commands:
OtherDB.connection.execute("select count(*) from users")