Extract entities from external source and manage it in my app

I'm facing a modelization problem where i can't find a satisfying solution.

My problem:

• I want to modelize a simple OneToMany relationship between two entities
• These entities are from an external source. I can get them in a tsv format, but i have to clean them before persisting
• My app is developped in Python, but since its more the concept/reflexion that I want to really understand, we can talk over language that are more object oriented

My Question:

I don't know how to build my classes to answer this need.

• Should I use a single class that extracts, get, query and updates my entity ?
• Should I use a repository pattern ? But since I have two entities and a relationship between them, how do I handle all the entities references in the Repository class ?
• Is there any other pattern that could suit my need ?

Thanks !