In order to enhance our database and come closer to MongoDB's functionality, we will consider adding two essential features - Schema Validation and Transaction Handling. These features are incumbent in large-scale applications, particularly in domains like finance and Artificial Intelligence.

Schema Validation is a way to ensure that your database only contains data that fits specific criteria. This could mean data types, lengths, or even semantic rules, improving the robustness of data quality and integrity.

Consider a situation where you're building a machine learning model using data from your document-oriented database. If there is no validation in place, you could end up training your model with incorrect or irrelevant data causing skewed results.

Transaction Handling ensures that your database remains in a consistent state, even in case of errors. In simpler terms, it follows the ACID principle - Atomicity, Consistency, Isolation, and Durability. Especially in finance, atomic transactions are critical. Failure in midway could lead from incorrect balances to potential legal issues.

To incorporate these features, we would extend our classes and functions. Let's start with schema validation, building on our existing Document and Collection classes.

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class Document:

    def __init__(self, doc_id, collection):

        self.doc_id = int(doc_id)

        self.collection = collection

    def validate_schema(self, data):

        # Code to validate the data against the pre-defined schema

        pass

​

class Collection:

    def __init__(self, name):

        self.name = name

        self.documents = {}

    def validate_transaction(self):

        # Transaction validation code

        pass

​

if __name__ == '__main__':

  transaction_data = {'type': 'purchase', 'amount': 100, 'currency': 'USD'}

  financial_collection = Collection('financialData')

  financial_doc = Document(1, financial_collection)

  # Implement the validate_schema method in the Document class

  financial_doc.validate_schema(transaction_data)

  # Implement the validate_transaction method in the Collection class

  financial_collection.validate_transaction()