Many to Many relationship in Firebase

Question

I have a Firebase database. I have Companies and Contractors. A Contractor can work for more than one Company and a Company can have multiple Contractors. This is a straightforward many to many relationship. I want to be able to answer the questions about Companies and Contractors:

1. Given a Company, who are the current Contractors.
2. Given a Contractor what Companies are they working for.

What are the alternatives for structuring the data within firebase?

Answer 1

The self-answer is indeed one way of modeling this. It's probably the most direct equivalent of how you'd model this in a relational database:

• contractors
• companies
• companyAndContractorsAssignment (the many-to-many connector table)

An alternative would be to use 4 top-level nodes:

• contractors
• companies
• companyContractors
• contractorCompanies

The last two nodes would look like:

companyContractors
    companyKey1
        contractorKey1: true
        contractorKey3: true
    companyKey2
        contractorKey2: true
contractorCompanies
    contractorKey1
        companyKey1: true
    contractorKey2
        companyKey2: true
    contractorKey3
        companyKey1: true

This bidirectional structure allows you to both look up "contractors for a company" and "companies for a contractor", without either of these needing to be a query. This is bound to be faster, especially as you add contractors and companies.

Whether this is necessary for your app, depends on the use-cases you need, the data sizes you expect and much more.

Recommended reading NoSQL data modeling and viewing Firebase for SQL developers. This question was also featured in an episode of the #AskFirebase youtube series.

Update (2017016)

Somebody posted a follow-up question that links here about retrieving the actual items from the "contractors" and "companies" nodes. You will need to retrieve those one at a time, since Firebase doesn't have an equivalent to SELECT * FROM table WHERE id IN (1,2,3). But this operation is not as slow as you may think, because the requests are pipelined over a single connection. Read more about that here: Speed up fetching posts for my social network app by using query instead of observing a single event repeatedly.