Delay DB connecting until it becomes necessary

[a2ikm]

Currently Tako establishes connections with shards at booting up.
This PR changes it to at executing first query.

Furthermore, this enables refreshing connections by each request with activerecord-refresh_connection.

Note that these changes are made by @hoco, who is my co-worker.
Thanks @hoco!

[the40san]

🤔
Should we really need refresh connection by each request?
Fmm? Perhaps it is Architecture or resource-dependent issue.

[the40san]

btw connection timing change is good idea.

[the40san]

Does it makes any change? :)

[a2ikm]

Please note that calling ActiveRecord::ConnectionHandling#connection connects with the database.

Currently Tako::Repository#add calls it.
With these changes, Tako::Repository#add doesn't, but Tako::Repository#create_proxy does.
So a connection is created at querying with a new proxy.

[the40san]

Do we need alias_method for that?

[a2ikm]

Ah, I see what you mean.
Yes, we need it because the connection argument provided to Tako::Proxy#initialize can be affected by Tako::ProxyStack.top.

With the original code, temporary_class.connection in add method always returns the result of super in Tako::ActiveRecordExt::ConnectionHandling#connection because Tako::ProxyStack.top is nil when loading shards.yml.

module Tako
  module ActiveRecordExt
    module ConnectionHandling
       def connection
         return Tako::Repository.create_proxy(@force_shard).connection if force_shard?
         if Tako::ProxyStack.top
           Tako::ProxyStack.top.connection
         else
           super # Always comes here when called within `add` method
         end
       end

With the new code, proxy_classes[shard_name.to_sym].connection in create_proxy method can return a result of both super or Tako::ProxyStack.top.connection because Tako::ProxyStack.top can be non-nil. If it returns latter, shard's name and real connection conflict in a proxy object. This is same if force_shard? is true .

For example, without the alias, this code creates a proxy object where shard_name is :shard02 but connection is to shard01.

Tako.shard(:shard01) { ModelA.shard(:shard02).where(value1: 1).limit(5).count }

So we need to call the original connection method to reduce effects of Tako::ProxyStack.top.

[the40san]

Ahhhh!!! I got understand. In the sample case.

Tako.shard(:shard01) { ModelA.shard(:shard02).where(value1: 1).limit(5).count }

if ModelA.shard(:shard02) is first query, it might be go to :shard01 because of nested w/o alias...
Must be fixed.

and there are some refactor-able magic code...!

[the40san]

Thanks I understood whole things

[the40san]

👍

[a2ikm]

Thank you!

there are some refactor-able magic code...!

Yeah, I think so too :)