At what point does ActiveRecord actually connect to the database? - mysql

Question

I have a ruby script that is using ActiveRecord (2.3.12) to access a MySQL database. The flow goes something like, "read database values from a config file", "connect to database", "Create table A if it doesn't exist", "download and parse a file", "save parsed records to A".
The code looks like the following:
ActiveRecord::Base.establish_connection(
:adapter => 'mysql',
:database => database_name,
:username => username,
:password => password,
:host => "localhost",
:port => 3306
)
...
ActiveRecord::Schema.define do
create_table a, :force => true do |t|
t.string :last_name, :limit => 60, :default => "", :null => false
t.string :first_name, :limit => 30, :default => "", :null => false
t.string :middle_initial, :limit => 2, :default => ""
t.string :dob, :limit => 12, :default => "", :null => false
end
end unless A.table_exists?
However, if I put incorrect DB credentials, or a non-existent database name into the establish_connection method, the script doesn't seem to give any errors or throw any exceptions until I actually try to perform some operation on the database (i.e., create table A). I tried a begin-rescue-end around establish_connection, but it never went into the rescue block.
Why does establish_connection seem to not really...well...establish the connection? And for the life of me, I can't figure out what it is even supposed to return. The docs HERE sure don't seem to be any help.
Or am I doing something wrong? Please help!

Answer 1

I usually use ActiveRecord::Base.connection.active? statement to check the whether the ActiveRecord is really connected to database.
def establish_database_connection
begin
ActiveRecord::Base.establish_connection config["database"]
ActiveRecord::Base.connection.active?
logger.info "Connected to Database"
rescue Exception => e
logger.error "Exception db connection : #{e.message} "
raise "Database connection failed"
end
end
Without ActiveRecord::Base.connection.active? statement the above code won't raise any error on invalid credentials.

Answer 2

RSK's solution will leave a connection checked out for the current thread. If you don't want that, try this (adapted from https://gist.github.com/limhoff-r7/71ee6b1568b604e131a8, which is for Postgres only):
ActiveRecord::Base.establish_connection
# Check if the spec passed to `ActiveRecord::Base.establish_connection` can connect to the database.
#
# #return [true] if an active connection can be made to the database using the current config.
# #return [false] if an active connection cannot be made to the database.
def connection_established?
begin
# use with_connection so the connection doesn't stay pinned to the thread.
ActiveRecord::Base.connection_pool.with_connection {
ActiveRecord::Base.connection.active?
}
rescue Exception
false
end
end

Answer 3

I agree with #Luke Imhoff: Connections that are manually checked out from ActiveRecord's connection-pool, have to manually be returned to the pool.
As a note, however, I suggest to use the connection that ActiveRecord yields to the block
ActiveRecord::Base.connection_pool.with_connection { |con| con.active? }
Referring to the documentation of :with_connection:

Answer 4

I'm not an expert, but I always assumed that establish_connection was more of a connection definition, whereas the actual connection is made when it is used, in this case, when the table_exists? runs