> For the complete documentation index, see [llms.txt](https://romebell.gitbook.io/sei-412/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://romebell.gitbook.io/sei-412/databases/mongodb/intermediate.md).

# 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:

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

There are several comparisons we can perform

* $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

```javascript
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 &&)

```javascript
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 ||)

```javascript
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.

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

## Explain intermediate update operators

We can increase a specific value

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

Multiple a value

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

Push a value onto an array

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

Pop a value off an array

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

Remove a property altogether

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

Rename a field

```javascript
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:

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

Upsert:

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