how to read back result when using mongodb transaction? - mongodb

Question

All mongodb transaction examples I have seen so far don't allow reading results back. For example (pseudo code):
begin_transaction
collection_one.update(...)
collection_two.update(...)
commit_transaction
The problem is, what if I want to update collection_two based on the result of updating collection_one?
For example?
begin_transaction
result = collection_one.update(...)
if (result.update_count() > 0)
{
collection_two.update(...)
}
commit_transaction
I have never seen an example like the above? It seems that when use transaction, I can't get the result back.
Another example,
begin_transaction
result = collection_children.find({name: 'xxx'})
collection_team.delete({name in result})
commit_transaction
Basically, I want to perform a find on a collection, and based the find result to perform a second action on a different collection.
And I want the 2 actions together be atomic.

Answer 1

Here is an example of how this works as expected with Mongoose. The same example obviously is possible without Mongoose.
var author = new Author({ email: "test22#test.com", other: [{a: 1}});
var book = new Book({ title: 'ABC' })
let doFoo = async () => {
const session = await mongoose.startSession();
session.startTransaction();
try {
const opts = { session, new: true };
let _author = await author.save() // Notice we get back the record
let _book = await book.save()
// Notice we are using now the new id of the just saved record
await Author.findOneAndUpdate({ _id: _author.id }, { $set: { other: { foo: 2 } }}, opts);
await Book.findOneAndUpdate({ _id: _book.id }, { $set: { title: "ABC" }}, opts);
await session.commitTransaction();
session.endSession();
} catch (error) {
await session.abortTransaction();
session.endSession();
throw error; // Rethrow so calling function sees error
}
}
doFoo()
So in the example above we create/save two different records in their respective collections and after that based on the new records we go back and update them.