===== Logging turtlesim data =====
This is a simple example to show logging of pose and velocity information from running the ''turtlesim'' node.

==== Logging the data ====
Start the turtlesim and log all the topics using the following sets of commands
  rosrun turtlesim turtlesim_node
  rosrun turtlesim turtle_teleop_key # for keyboard teleop to get changing data
  rosrun mongodb_log mongodb_log -a # for all topics. check help for more options
  # use Ctrl + C to stop
  
==== Accessing the data ====
The data that has been logged can be accessed via may ways, directly using terminal mongo client or via the language APIs. These are each shown below;

=== Via the mongo terminal client ===
This is installed when you get the mongodb package installed on the system. 
   mongo # alternatively, use the ros version with rosrun mongodb mongo
   # you should see an interface like the following
   MongoDB shell version: 2.0.4
   connecting to: test
   # change to the ros database using 
   > use roslog
   switched to roslog
   # see a list of databases using
   > show dbs
   local	(empty)
   roslog	0.203125GB
   # see the list of items stored using the following
   > db.getCollectionNames()
   [
    	"system.indexes",
    	"turtle1_color_sensor",
    	"turtle1_command_velocity",
    	"turtle1_pose"
   ]
   # further commands are given in standard mongo cheatsheets
   > 

=== Via a python API ===
The full documentation of the python API is available online at [[http://api.mongodb.org/python/current/|http://api.mongodb.org/python/current/]]. Below is a simple example usage:
  import pymongo
  mclient = pymongo.Connection()
  db = mclient['roslog']
  # get the collections
  db.collection_names()
  [u'turtle1_color_sensor', u'system.indexes', u'turtle1_pose', u'turtle1_
  command_velocity']
  # show all the data in the database
  docs = db['turtle1_pose'].find()
  for d in docs:
    print d
  # this displays a long list with elements like these
  {u'__topic': u'/turtle1/pose', u'linear_velocity': 0.0, u'theta': 1.9824
  440479278564, u'y': 8.26992130279541, u'x': 4.029432773590088, u'_id': O
  bjectId('51c98fcca6c646570b000901'), u'angular_velocity': 0.0, u'__recor
  ded': 1372164044.44448}
  {u'__topic': u'/turtle1/pose', u'linear_velocity': 0.0, u'theta': 1.9824
  440479278564, u'y': 8.26992130279541, u'x': 4.029432773590088, u'_id': O
  bjectId('51c98fcca6c646570b000902'), u'angular_velocity': 0.0, u'__recor
  ded': 1372164044.460494}
  
  # querying can be done directly on these. e.g.
  q = db['turtle1_pose'].find({'__recorded':{'$gt' : 1372164044.44448}}) 
  q.count()
  1 # which is exactly the last recording as shown in the data snippet above