12.3 MongoDB Style Guide

PostgreSQL vs. MongoDB¶

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.

• STRONG SUGGESTION : Wrap all field names in double-quotations (" ") to avoid syntax errors while working with PyMongo.

• 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 $¶

1. 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 } });

2. Aggregation Operators
   In MongoDB’s aggregation framework, $ is often used to refer to specific stages, operators, and fields.

   • $match, $group, $sort, $project, etc.

3. 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

4. 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"