require 'thread'
require 'mongoid/collection_proxy'

#
# Proxy that enables runtime swapping of a MongoDB database, as it
# appears to be cached at several points within Mongoid.
#
# This proxy is generated by the ConnectionProxy when asked for a
# specific database.
#
class DatabaseProxy
    #
    # Semaphore for preventing badness when multithreading.
    #
    @mutex = Mutex.new

    #
    # All connections that we know about, and then the databases tied to
    # them get stored in here.
    #
    @pool = Hash.new

    #
    # Accessor for class-level instance variable that holds all the
    # connection-database pairs that we know about, so that we can
    # switch all of them globally.
    #
    def DatabaseProxy.pool
        @pool
    end

    #
    # Global mutex... for great thread safety!
    #
    def DatabaseProxy.mutex
        @mutex
    end

    #
    # Set our default connection and name.
    #
    def initialize(connection, name)
        @connection = connection
        switch(name)
    end

    #
    # Convenience method for synchronizing threads.
    #
    def synchronize(&block)
        DatabaseProxy.mutex.synchronize(&block)
    end

    #
    # Switch to a different database on the same connection.
    #
    def switch(name)
        synchronize do
            # Connect to the cached database if available.
            pool = (DatabaseProxy.pool[@connection] ||= {})
            database = (pool[name] || (pool[name] = @connection.db(name)))

            # Set the default if we haven't, and away we go (for this
            # thread only).
            @default ||= database 
            Thread.current[:mongo_database] = database
        end
    end

    #
    # Resets back to the default database connections.
    #
    def reset!
        synchronize { Thread.current[:mongo_database] = nil }
    end

    #
    # Returns the raw Mongo::DB object.
    #
    def target
        synchronize { Thread.current[:mongo_database] || @default }
    end

    #
    # Create a proxied collection.
    #
    def create_collection(name, opts)
        CollectionProxy.new(self, name, opts)
    end

    #
    # Proxy methods to the correct database.
    #
    def method_missing(*args, &block)
        target.send(*args, &block)
    end
end