The server_block extension adds the Database#with_server method, which takes a shard argument and a block, and makes it so that access inside the block will use the specified shard by default.
First, you need to enable it on the database object:
DB.extension :server_block
Then you can call with_server:
DB.with_server(:shard1) do DB[:a].all # Uses shard1 DB[:a].server(:shard2).all # Uses shard2 end DB[:a].all # Uses default
You can even nest calls to with_server:
DB.with_server(:shard1) do DB[:a].all # Uses shard1 DB.with_server(:shard2) do DB[:a].all # Uses shard2 end DB[:a].all # Uses shard1 end DB[:a].all # Uses default
Note this this extension assumes the following shard names should use the server/shard passed to with_server: :default, nil, :read_only. All other shard names will cause the standard behavior to be used.