# * George Moschovitis  <gm@navel.gr>
# (c) 2004-2005 Navel, all rights reserved.
# $Id: backend.rb 202 2005-01-17 10:44:13Z gmosx $

require 'yaml'

require 'og/connection'

class Og

# Abstract backend. A backend communicates with the RDBMS.
# This is the base class for the various backend implementations.

class Backend
	
	# The actual connection to the database
	attr_accessor :conn

	# Intitialize the connection to the RDBMS.
	 	
	def initialize(config)
		raise "Not implemented"
	end

	# Close the connection to the RDBMS.
	 	
	def close()
		@conn.close()
	end	

	# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
	# :section: Utilities
	# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

	# Encode the name of the klass as an sql safe string.
	# The Module separators are replaced with _ and NOT stripped 
	# out so that we can convert back to the original notation if 
	# needed. The leading module if available is removed.
	
	def self.encode(klass)
		"#{klass.name.gsub(/^.*::/, "")}".gsub(/::/, "_").downcase
	end

	# The name of the SQL table where objects of this class 
	# are stored.
	
	def self.table(klass)
		"_#{Og.table_prefix}#{encode(klass)}"
	end

	# The name of the join table for the two given classes.
	
	def self.join_table(klass1, klass2)
		"_#{Og.table_prefix}j_#{encode(klass1)}_#{encode(klass2)}"
	end
	
	# Returns the props that will be included in the insert query.
	# For some backends the oid should be stripped.
	
	def self.props_for_insert(klass)
		klass.__props
	end
	
	# Precompile the insert code for the given class.
	# The generated code sets the oid when inserting!
	
	def self.eval_og_insert(klass)
		props = props_for_insert(klass)
		
		values = props.collect { |p| write_prop(p) } 
		
		sql = "INSERT INTO #{table(klass)} (#{props.collect {|p| p.name}.join(',')}) VALUES (#{values.join(',')})"

		if klass.instance_methods.include?("og_pre_insert")
			pre_cb = "og_pre_insert(conn);"
		else
			pre_cb = ""
		end

		if klass.instance_methods.include?("og_post_insert")
			post_cb = "og_post_insert(conn);"
		else
			post_cb = ""
		end

		if klass.instance_methods.include?("og_pre_insert_update")
			pre_cb << "og_pre_insert_update(conn);"
		end

		if klass.instance_methods.include?("og_post_insert_update")
			post_cb << "og_post_insert_update(conn);"
		end
		
		klass.class_eval %{
			def og_insert(conn)
				#{insert_code(klass, sql, pre_cb, post_cb)}
			end
		}
	end

	# Precompile the update code for the given class.
	# Ignore the oid when updating!
	
	def self.eval_og_update(klass)
		props = klass.__props.reject { |p| :oid == p.symbol }
		
		updates = props.collect { |p|
			"#{p.name}=#{write_prop(p)}"
		}

		sql = "UPDATE #{klass::DBTABLE} SET #{updates.join(', ')} WHERE oid=#\{@oid\}"

		if klass.instance_methods.include?("og_pre_update")
			pre_cb = "og_pre_update(conn);"
		else
			pre_cb = ""
		end

		if klass.instance_methods.include?("og_post_update")
			post_cb = "og_post_update(conn);"
		else
			post_cb = ""
		end

		if klass.instance_methods.include?("og_pre_insert_update")
			pre_cb << "og_pre_insert_update(conn);"
		end

		if klass.instance_methods.include?("og_post_insert_update")
			post_cb << "og_post_insert_update(conn);"
		end
		
		klass.class_eval %{
			def og_update(conn)
				#{pre_cb}
				conn.exec "#{sql}"
				#{post_cb}
			end
		}
	end

	# Precompile the code to read objects of the given class 
	# from the backend. In order to allow for changing 
	# field/attribute orders we have to use a field mapping hash.
	
	def self.eval_og_deserialize(klass, og)
		calc_field_index(klass, og)
		
		props = klass.__props 
		code = []
		
		props.each do |p|
			if idx = og.managed_classes[klass].field_index[p.name]
				# more fault tolerant if a new field is added and it 
				# doesnt exist in the database.
				code << "@#{p.name} = #{read_prop(p, idx)}"
			end
		end
		
		klass.class_eval %{
	 		def og_deserialize(res, tuple = nil)
				#{code.join('; ')}
			end
		}
	end				
	
	# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
	# :section: Connection methods. 
	# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

	# Create the database.
	
	def self.create_db(database, user = nil, password = nil)
		Logger.info "Creating database '#{database}'."
	end
	
	# Drop the database.
	
	def self.drop_db(database, user = nil, password = nil)
		Logger.info "Dropping database '#{database}'."
	end
	
	# Execute an SQL query and return the result. 
	
	def query(sql)
		raise "Not implemented"
	end
	
	# Execute an SQL query, no result returned.
	
	def exec(sql)
		raise "Not implemented"
	end

	# Execute an SQL query and return the result. Wrapped in a 
	# rescue block. 
	
	def safe_query(sql)
		raise "Not implemented"
	end
	
	# Execute an SQL query, no result returned. Wrapped in a 
	# rescue block.
	
	def safe_exec(sql)
		raise "Not implemented"
	end
	
	# Check if it is a valid resultset.
	
	def valid?(res)
		raise "Not implemented"
	end

	# Start a new transaction.
	
	def start
		exec "START TRANSACTION"
	end
	
	# Commit a transaction.
	
	def commit
		exec "COMMIT"
	end
	
	# Rollback transaction.
	
	def rollback
		exec "ROLLBACK"
	end	

	# Create the fields that correpsond to the klass properties.
	# The generated fields array is used in create_table.
	# If the property has an :sql metadata this overrides the 
	# default mapping. If the property has an :extra_sql metadata 
	# the extra sql is appended after the default mapping.
	
	def create_fields(klass, typemap)
		fields = []
		
		klass.__props.each do |p|
			klass.sql_index(p.symbol) if p.meta[:sql_index]
				
			field = "#{p.symbol}"
			
			if p.meta and p.meta[:sql]
				field << " #{p.meta[:sql]}"
			else
				field << " #{typemap[p.klass]}"
				# attach extra sql
				if p.meta and extra_sql = p.meta[:extra_sql]
					field << " #{extra_sql}"
				end
			end
			
			fields << field 
		end

		return fields
	end

	# Create the managed object table. The properties of the
	# object are mapped to the table columns. Additional sql relations
	# and constrains are created (indicices, sequences, etc).
	
	def create_table(klass)
		return if query("SELECT * FROM #{klass::DBTABLE} LIMIT 1")
	end
	
	# Drop the managed object table
	
	def drop_table(klass)
		exec "DROP TABLE #{klass::DBTABLE}"
	end

	# Deserialize one row of the resultset.
	
	def deserialize_one(res, klass)	
		raise 'Not implemented'
	end
	
	# Deserialize all rows of the resultset.
	
	def deserialize_all(res, klass)	
		raise 'Not implemented'
	end
	
	# Return a single integer value from the resultset.
	
	def get_int(res, idx = 0)
		raise 'Not implemented'
	end

end

end