module MongoMapper::Migration
  class MigrationDefinition
    def upgrade &block
      if block
        @upgrade = block
      else
        @upgrade
      end
    end
    alias_method :up, :upgrade
    
    def downgrade &block
      if block
        @downgrade = block
      else
        @downgrade
      end
    end
    alias_method :down, :downgrade
  end
  
  class << self
    def define_migration version, &block
      version.should! :be_a, Integer
      definition = MigrationDefinition.new
      block.call definition
      migration_definitions[version] = definition
    end
    
    def define_global_migration version, &block
      version.should! :be_a, Integer
      definition = MigrationDefinition.new
      block.call definition
      global_migration_definitions[version] = definition
    end
        
    def update_global_database_to version  
      puts "Migration for Global Database:"
      update_database MongoMapper.global_database, global_migration_definitions, version
    end
    
    def update_database_to version      
      puts "Migration for Accounts Databases:"
      results = {}
      MongoMapper.accounts_names.each do |account_name|
        MongoMapper.account_database account_name do |db|
          result = update_database db, migration_definitions, version
          results[db.name] = result
        end
      end
      results
    end
    
    def metadata db
      col = db.collection 'db_metadata'
      metadata = (col.find_one || {:version => 0}).to_openobject
    end
    
    def global_migration_definitions; @global_migration_definitions ||= [] end
    def migration_definitions; @migration_definitions ||= [] end    
    
    protected
      def increase_db_version migration_definitions, db
        m = metadata(db)
        migration = migration_definitions[m.version + 1]        
        raise "No upgrade for version #{m.version + 1}" unless migration and migration.up
        
        migration.up.call
        
        m.version += 1
        update_metadata db, m
        
        puts "Database #{db.name} upgraded to version #{m.version}"
      end
      
      def decrease_db_version migration_definitions, db
        m = metadata(db)
        migration = migration_definitions[m.version]        
        raise "No downgrade for version #{m.version}" unless migration and migration.down
        
        migration.down.call
        
        m.version -= 1
        update_metadata db, m
        
        puts "Database #{db.name} downgraded to version #{m.version}"
      end
      
      def update_database db, migrations, version      
        if metadata(db).version == version
          puts "Database #{db.name} already is of #{version} version, no migration needed"
          return false
        end

        increase_db_version migrations, db while metadata(db).version < version
        decrease_db_version migrations, db while metadata(db).version > version
        true
      end
      
      def update_metadata db, metadata
        col = db.collection 'db_metadata'
        col.save metadata.to_hash
      end
          
      def reset
        [global_migration_definitions, migration_definitions].every.clear
      end
  end
end