What is the idiomatic way to implement foreign keys in CouchDB?

Question

Let me state a simple example: You have an Order and a Shopping Cart. One way I envision persisting this is to save an Order document and a Cart document. The Order document could have a field called "shopping-cart" whose value is the UUID of the relevant Cart document. Another way I can imagine doing this is to save an Order document with the "shopping-cart" field containing an associative array of the entire Cart. In other words, instead of saving the Cart explicitly as an independent document, I embed the Cart document in the Order document.

What if we decide later that a Cart should be persistent, so a returning user will find his half-finished Cart waiting for him across sessions? I imagine we could then combine both methods, keeping the Cart separate while it's incomplete and embedding it in the Order document when it's finalized/purchased.

Both methods would work, though I worry about CouchDB not having foreign key constraints; in the first method the Cart document could be deleted, leaving you with a corrupt data set.

How do you decide which method to use? Is one of these methods more idiomatic to CouchDB? Are there any methods I missed?

I'm new to CouchDB so it's difficult for me to see the advantages/disadvantages to having a more or less normalized structure.

Answer 1

If on the other hand they aren't one to one then using a complex key you can use view collation to do a join.

function(doc) {
  if (doc.type == 'order') {
    emit([doc.cartid, 1], doc);
  } else if (doc.type == 'cart') {
    emit([doc.id, 0], doc);
  }
}

the docs will be collated by cartid with the orders coming after the cart. Your application code can easily join this stream and you can query by a particular cartid using startkey and endkey.

see: View Collation for the collation rules.

You can also use a reduce to join them together as well.

Just change the map function to this:

function(doc) {
  if (doc.type == 'order') {
    emit([doc.cartid, 1], {cartid: doc.cartid, orders: [doc]});
  } else if (doc.type == 'cart') {
    emit([doc.id, 0], {cartid: doc.id, orders: [], cart: doc);
  }
}

and add a reduce function like this:

function(keys, values) {
  var out = {cartid: null, orders: [], cart: null};
  for (idx in values) {
    var doc = values[idx];
    out['cartid'] = doc.cartid;
    if (doc.cart) { out['cart'] = doc.cart };
    for (idx2 in doc.orders) {
      out.orders.push(doc.orders[idx2]);
    }
  }
  return out;
}

This will return a single document per cart which will a cartid, an array of order documents and a cart document.

Apologies if there are errors in the above code but I don't have a couchdb test instance handy to try them out. You should get the general idea though and the CouchDB wiki has more details.