Intermediate Mongo

Lesson Objectives

Let's go into a few more complex examples of how Mongo can be used. We'll cover the following:

1. Using intermediate find operators

2. Explaining update operators

3. Explaining upserts

4. Explaining multiple updates

Use intermediate find operators

To find all the documents in our collection that have a weight property greater than 700, we run:

db.employees.find({
   weight: { $gt : 700 }
});

There are several comparisons we can perform

Syntax	Meaning
$lt	less than
$lte	less than or equal to
$gt	greater than
$gte	greater than or equal to
$ne	not equal to
$exists	does the property exist on an object -- either true or false
$in	does the value exist within the given array

If the field is an array, you can search for a match within that array

db.employees.find({ loves: 'energon' });

If the object you pass into find() has more than one attribute, it will return documents that match both criteria. This is called an AND statement (like &&)

db.employees.find({
   gender: 'm',
   weight: { $gt: 700 }
});

To find all documents that match at least one of a set of criteria, use an OR statement (like ||)

db.employees.find({
   $or: [
      { loves: 'apple'},
      { weight: { $lt: 500 }
   ]
});

To find documents that have a value that matches multiple criteria, pass an object that contains both tests. This is similar to an AND (&&), but for one property. If you try to do a normal AND statement, but use the same property, twice it won't work.

db.employees.find({
   salary: {
      $gte : 80,
      $lte : 165
   }
});

Explain intermediate update operators

We can increase a specific value

db.employees.update({ name: 'Pilot'}, { 
   $inc: { salary: -2 }
});

Multiple a value

db.employees.update({ name: 'Pilot'}, {
   $mul: { salary: (1/2) }
});

Push a value onto an array

db.employees.update({ name: 'Aurora' }, {
   $push: { loves: 'sugar' }
});

Pop a value off an array

db.employees.update({ name: 'Aurora' }, {
   $pop: { loves: 1 }
});

Remove a property altogether

db.employees.update({ name: 'Aurora' }, {
   $unset: { loves: '' }
});

Rename a field

db.employees.update({ name: 'Aurora' }, {
   $rename: { wrong_field_name : 'correct_field_name' }
});

For more operators, look here: http://docs.mongodb.org/manual/reference/operator/update/#update-operators

Explain upserts

Upserts will insert a value if it doesn't exist. If it does, it will update it.

Normal:

db.hits.update({ page: 'employees' }, {
   $inc: { hits: 1 }
});

db.hits.find();

Upsert:

db.hits.update({ page: 'employees' }, {
   $inc: { hits: 1 }
},
{
   upsert:true
});

db.hits.find();

Another Upset:

db.hits.update({ page: 'employees' },{
   $inc: { hits: 1 }
},
{
   upsert:true
});

db.hits.find();

Summary

MongoDB is quite versatile and gives the developer a lot more freedom than SQL. What would easily break a SQL database - like suddenly removing or renaming a column - are totally fine in a Mongo database!