CODEWITHSUNDEEP
mongodb
Matteo Calò
Matteo Calò

Reputation: 351

How to use MongoDB maintaining integrity between documents?

I'm writing an application, and I want to use MongoDB. In the past I've always used relational databases. I don't understand how I can maintain integrity. I explain me better with an example:

I have "Restaurants", "Dishes" and "Ingredients". If I create a document in MongoDB for a dish and it has many ingredients (array of object "ingredient"). I have also a collection with all ingredients.

If I change the name of ingredient, how can I update the ingredient's name into "dish" document?

Upvotes: 1

Views: 504

Answers (1)

Markus W Mahlberg
Markus W Mahlberg

Reputation: 20683

Your description sounds like an artificial example and therefor is a bit hard to answer correctly, but I'll stick with it for now.

In your example, ask yourself if you really needed the ingredients to be unique. How would a user add a new one? Would he or she have to search the ingredients collection first? Does it really make a difference wether you have two or three instances of bell peppers in your database? What about qualifiers (like "pork loin" or "Javan vanilla" or "English cheddar"). How would be the usability for that?

The NoSQL approach would be

Ah, screw it! Let's suggest ingredients other users entered before. If the user chooses one, that's fine. Otherwise, even if every dish has it's own ingredient list, that's fine, too. It'd accumulate to a few megs at most.

So, you'd ditch the ingredient collection altogether and come up with a new model:

{
  _id: someObjectId,
  name: "Sauerbraten",
  ingredients: [
    {name: "cap of beef rump", qty: "1 kg"},
    {name: "sugar beet syrup", qty: "100ml"},
    {name: "(red) wine vinegar", qty: "100 ml"}
    {name: "onions", qty: "2 large" },
    {name: "carrots", qty: "2 medium"}
    // And so on
  ]
}

So, you don't need referential integrity any more. And you have the freedom for the user to qualify the ingredients. Now, how would you create the ingredient suggestions? Pretty easy: run an aggregation once in a while.

db.dishes.aggregate([
  {"$unwind":"$ingredients"},
  {"$group": {"_id":"$ingredients.name","dishes":{"$addToSet":"$_id"} }},
  {"$out":"ingredients"}
])

Resulting in a collection called ingredients, with the ingredient's names being indexed by default (since they are the _id). So if a user enters a new ingredient, you can suggest for autocomplete. Given the user enters "beef", your query would look like:

db.ingredients.find({"_id": /beef/i})

which should return

{ "_id": "cap of beef rump", "dishes": [ someObjectId ]}

So, without having referential integrity, you make your application easier to use, maintain and even add some features for basically free.

Upvotes: 1

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