Many-to-Many

A many-to-many relation joins two tables through a third one — the junction table.

Example

Products belong to many categories. Categories contain many products. The junction table is product_categories.

Protocol

"products": {
  "relations": {
    "categories": {
      "type":       "mtm",
      "entity":     "categories",
      "prototable": "categories",
      "through":    "product_categories",
      "fkfield":    "id:product_id:category_id:id"
    }
  }
}

The fkfield is four-part:

Position	Meaning
1	Local table column joining to the junction table
2	Junction column matching the local row
3	Junction column matching the remote row
4	Remote table column joined from the junction

So this reads as: products.id ↔ product_categories.product_id, then product_categories.category_id ↔ categories.id.

Query

mydb.products{
  id,
  name,
  categories{name}
}

Response shape

Like otm, the related side is an array:

{
  "products": [
    {
      "id": 1,
      "name": "Phone",
      "categories": [
        { "name": "Electronics" },
        { "name": "Mobile" }
      ]
    }
  ]
}

When to use it

• Products ↔ categories
• Students ↔ courses
• Posts ↔ tags
• Users ↔ roles

Note on the junction table

The junction table just needs to exist with the two foreign-key columns. ONQL doesn't care if it has extra columns (e.g. created_at, added_by) — they're invisible unless you declare a separate entity for the junction itself.