title: How to insert document or fields in nested documents in mongo
body: <p>I want collection named systems under which there is a document named sub_systems and under which there are three documents named high mid &amp; low. And each high, mid and low contains separate multiple documents Like below</p>
  
  <pre><code>systems: {
             sub_system_1 : {
                              high :{ {task1},
                                      {task2},
                                      .......
                                    },                                
                              mid  :{ {task1},
                                      {task2},
                                      .......
                                    },
                              low :{ {task1},
                                      {task2},
                                      .......
                                    },
                            },
             sub_system_2 : {
                              high :{ {task1},
                                      {task2},
                                      .......
                                    },                                
                              mid  :{ {task1},
                                      {task2},
                                      .......
                                    },
                              low :{ {task1},
                                      {task2},
                                      .......
                                    },
                            },
             ............
           }
  </code></pre>
  
  <p>So following are the questions I'm having
  1. How will I create such nested document.
  2. How will I insert new sub_system in the systems collection.
  3. How will I insert new priority (high, mid, low) under sub_system document.
  4. How will I insert new task in a specific sub_system and  under a specific priority(high,low, mid) document</p>
  
  <p>I want to know this using mongo shell and also with perl mongodb</p>