MongoDb updating old documents - mongodb

Question

I am new to MongoDb and to the NoSQL concept of storing information. Unlike in SQL Server, adding or updating a column name in a table will affect all of the records. I can assume that when I pull a record, that column will be there. It may or may not have a value depending on how it was setup.
But how would you handle a situation in MongoDb where you have an outdated document and you want to pull it and now your code is looking for a property that did not exist or was just recently renamed?

Answer 1

There are few examples of different technics.
Assuming we have a collection pets with documents like:
{
kind: "cat",
age: 2
}
and at some point we added a field "nick", so new documents look like:
{
kind: "cat",
age: 2,
nick: "Tom"
}
Your app require "nick" field to list pets.
Use defaults
Update old documents with default values (you may known it as 'ALTER TABLE' magic in SQL terms):
db.pets.updateMany({nick: {$exists: false}}, {$set: {nick: "NONAME"}});
If you need to support both versions you need to do it runtime.
On application side:
db.pets.find({}).forEach(pet => print(p.nick || "NONAME") );
On db side:
db.pets.aggregate([
{$project: {
kind: 1,
age: 1,
nick: { $ifNull: [ "$nick", "NONAME" ] }
}}
])
Ignore invalid documents
Remove them:
db.pets.remove({nick: {$exists: false}})
If you need to support both versions you need to filter them out runtime:
db.pets.find({
kind: {$exists: true},
age: {$exists: true},
nick: {$exists: true}
);
You can make it more defensive by specifying type:
db.pets.find({
kind: {$type: "string"},
age: {$type: "int"},
nick: {$type: "string"}
);
Stop program execution
db.pets.find({}).forEach(pet => if(!pet.nick){throw new Error("Pet " + pet._id + " has no nick!")} );