Add ORM section.

Author: louisrli

module2-database-technologies/README.md

@@ -288,17 +288,65 @@ know how to query both in command-line and graphically.
     through the command-line or JavaScript)
 
 ## ORMs
+ORM stands for **object-relational mapping**. Don't let the terminology scare
+you: an ORM is simply a library that makes it easier to communicate with a
+database in code. For example, some popular JavaScript ORMs are
+[Sequelize](https://sequelize.org/), [TypeORM](https://typeorm.io/#/), etc.
+These are all libraries that help you query the database from the code.
+
+Why is an ORM useful? Consider the case of a SQL database. Writing raw SQL
+queries in the code is quite unsafe -- there is no type-checking on raw query
+strings, and a single typo could cause the query to fail. Additionally,
+substituting variable values into a query (such as `SELECT * FROM customer WHERE
+first_name = $myVariable`) can get quite cumbersome if you are manipulating the
+strings yourself.
+
+ORMs often simply the process of fetching relations as well. For example, if you
+want to get all the posts for a user, you don't have to think so hard about
+concepts such as joins. The same goes at insertion time: you don't have to think
+too hard about foreign keys. 
+
+Here is an example from another Node ORM, Prisma:
+```
+const createCategory = await prisma.post.create({
+  data: {
+    title: 'How to be Bob',
+    categories: {
+      create: [
+        {
+          assignedBy: 'Bob',
+          assignedAt: new Date(),
+          category: {
+            create: {
+              name: 'New category',
+            },
+          },
+        },
+      ],
+    },
+  },
+});
+```
+
+Notice how this conveniently creates rows in two tables in a single function
+call -- it creates a post, but it also creates a category for that post, and
+finally, it assigns that category to the post. This happens all in a single
+function call, in a way that feels natural to a JavaScript programmer (using
+objects, functions, and so on, rather than SQL queries).
 
-### ORM use cases
 
-Discuss why ORMs exist, drawbacks, etc
-Code along tutorial: Install sequelize/mongoose and do basic queries to see the benefits of an ORM
-Describe what happens when you need to update a database. Most ORMs have some concept of migrations so it is a bit important.
+### Drawbacks of ORMs
+For most use cases, ORMs are extremely convenient and should be used so that the
+code is more organized. However, ORMs can be inconvenient in the case of
+extremely complex queries, and it's possible that you may run into performance
+problems with certain queries that may be difficult to optimize.
+
+However, almost all ORMs allow you to use raw queries as a last resort (e.g.,
+type your own SQL and pass it to a function call) to handle these cases.
+
 ### ORM lab
 TODO
 
-## Database migrations
-
 ## Other popular databases
 
 ### Elasticsearch