MongoDB Style Guide

MongoDB Style Guide#

PostgreSQL vs. MongoDB#

	PostgreSQL	MongoDB
Definition	Object-relational database management system	Document database that stores data as key-value pairs
Schema	Each row represents a data point and there is a predetermined schema	Each document can hold various types of data without a predetermined schema
Basic Unit of Storage	Row (Tuple)	Serialized JSON document
Foreign Key Relations	Define relationships between tables using foreign keys	Does not use predefined relationships between collections

MongoDB Query Syntax#

Good structure for your queries will help make them easier to read.

For example, indenting and separating pairs of brackets:

db.orders.insertMany( [
   { "_id": 0, "name": "Pepperoni", "size": "small", "price": 19,
     quantity: 10},
   { "_id": 1, "name": "Pepperoni", "size": "medium", "price": 20,
     "quantity": 20},
] )

PyMongo#

PyMongo is a Python library that provides an interface for interacting with MongoDB from Python code.

All field names must be wrapped in double-quotations (" ") when using PyMongo because all field names are strings.
Note that because of this rule, the distiction between a <field> "name" and a string "Pepperoni" can be often difficult to find.

Use cases of $#

Indication of a Query Operator
$ symbol is often used to indicate query operators that help filter documents in a collection.
- $eq, $gt / $gte, $lt / $lte, $in / $nin, $exists, $and / $or, etc.
```
db.collection.find({ "age": { "$eq": 25 } });
```
```
db.collection.find({ "age": { "$gt": 25 } });
```
Aggregation Operators
In MongoDB’s aggregation framework, $ is often used to refer to specific stages, operators, and fields.
- $match, $group, $sort, $project, etc.
Referencing entire <field>s as an array
Depending on the operators used, you might want to use an entire <field> as an value array in the key-value pair operators. Then you would use $<field> instead of just <field>.
- For example:
```
db.orders.aggregate( [
{ "$match": { "status": 'A' } },
{ "$group": {
    "_id": "$cust_id",
    "total": { "$sum": "$price" }
    }
  }
] )
```
The query above wants to sum the price field for each unique cust_id with status A. So, the pipeline first filters out documents where the status field is A then groups based on the cust_id field to sum the price field.

Notice how when we are referring to the status field as a key to be filtered through we don’t use $. However, when we want to use the cust_id and price fields as values to be grouped on or summed, we use $cust_id and $price.

Below is the equivalent query in SQL:
```
SELECT cust_id, SUM(price) as total
FROM orders
WHERE status = 'A'
GROUP BY cust_id
```
Array Query Operator (Using $ in Arrays)
When working with arrays in MongoDB, the $ symbol can be used in special ways to match array elements in queries or update them.
- The positional $ operator is used to reference the first element in an array that matches the query condition when updating documents. It’s useful when you need to modify a specific element in an array.
```
db.collection.updateOne(
  { "comments.text": "Great post!" },
  { "$set": { "comments.$.status": "approved" } }
);
```
- This query finds the first element in the comments array where text field is "Great post!" and updates its status field to "approved"