> For the complete documentation index, see [llms.txt](https://timhwang21.gitbook.io/index/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://timhwang21.gitbook.io/index/programming/rails/cheap-many-to-many.md).

# Cheap many to many

There are two common ways to establish a many-to-many relationship:

1. `has_and_belongs_to_many`
2. `has_many, :through`

Both require a join table, and the second requires a little more setup than the first bit is more future-proof.

A third alternative is so simple it sounds dumb: simply add a JSON column to the parent that contains an array of child IDs. If you understand the constraints of this approach and your use case doesn't need them, it can be a flexible and noncommittal way to model relationships.

## Downsides

* No guarantee of data integrity (can have orphaned IDs, no `dependent: destroy`, etc.).
* Cannot access list of parents from child (can't see who a model belongs to).
* Worse performance (or, at least, unmanaged performance).

## Advantages

* Very noncommittal. When working with uncertainty, it's good to be as flexible as possible until you are sure of requirements. Creating a table is *very* committal.
* Automatic transactions. Simply update the parent record without needing to worry about data inconsistency.
* Can easily migrate to a true join-table based relationship.


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