Prototyping CouchDB Views (Take 2)

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I have CouchDB recipe documents that look like:

{
   "_id": "2008-07-10-choccupcake",
   "_rev": "1-1305119212",
   "prep_time": 10,
   "title": "Mini Chocolate Cupcakes with White Chocolate Chips",
   "published": true,
//...

   "date": "2008-07-10",
   "type": "Recipe",
//...
   "preparations": [
       {
           "brand": "",
           "quantity": 1,
           "unit": "cup",
           "order_number": 1,
           "description": "",
           "ingredient": {
               "name": "flour",
               "kind": "all-purpose"
           }
       },
       {
           "brand": "",
           "quantity": 0.5,
           "unit": "cup",
           "order_number": 2,
           "description": "",
           "ingredient": {
               "name": "sugar",
               "kind": "white, granulated"
           }
       },
//...
   ],
//...
}

With hundreds of recipes of this form, I would like a CouchDB view to give me a list of recipes by ingredient. Ultimately, I would like to generate a page like this (from the legacy Rails app):



I spiked this approach a while back. Looking back on that spike, I realize that I missed a couple of things: (1) the map function includes recipes that have not been published and (2) the reduce function does not deal with re-reduce (combining intermediate reduced steps).

The map that I used previously was:

function (doc) {
  for (var i in doc['preparations']) {
   var ingredient = doc['preparations'][i]['ingredient']['name'];
   var value      = [doc['_id'], doc['title']];
   emit(ingredient, value);
  }
}

That map function reads: for each preparation instruction, pull out the ingredient name and the document ID/title. The former is used to key the map-reduce. The latter will be used to create the links on the web page. As I mentioned this map function includes unpublished recipes. To exclude them, I need to add a simple conditional:

function (doc) {
  if (doc['published']) {
    for (var i in doc['preparations']) {
      var ingredient = doc['preparations'][i]['ingredient']['name'];
      var value      = [doc['_id'], doc['title']];
      emit(ingredient, value);
    }
  }
}

That produces results like this for the ingredient "apples":

"apples":
["2002-09-18-gingerapple", "Ginger Apple Crisps"],
"apples":
["2003-03-11-pecan", "Pecan Apple Tart"],
"apples":
["2003-12-07-applesauce", "Applesauce"],
"apples":
["2004-11-25-apple", "Apple Pie"]

To reduce that to a list of recipe IDs/title grouped by the "apples" ingredient, I had been using this function:

function(keys, values, rereduce) {
  return values;
}

This works for ingredients that are only in a few recipes (like "apples"):

"apple":[["2004-11-25-apple", "Apple Pie"],
 ["2003-12-07-applesauce", "Applesauce"],
 ["2003-03-11-pecan", "Pecan Apple Tart"],
 ["2002-09-18-gingerapple", "Ginger Apple Crisps"]]

For ingredients that are in lots of recipes, a single pass through the map function is not sufficient. Instead, CouchDB generates several arrays of the values (arrays of arrays in this case). Since I am doing nothing different when CouchDB calls my map function with the reduce flag set, I end up returning those arrays of arrays:

"butter": 
[[["2002-02-08-buffalo_chicken", "Buffalo Chicken Sandwich"],
  ["2002-02-13-mushroom_pasta", "Pasta with Mushroom Gruyere Sauce"],
  ["2002-02-17-sausage_pie", "Chicken Sausage Pot Pie"],
  ["2002-03-12-asparagus_omelet", "Asparagus Omelet"],
  ["2002-03-12-cinnamon_toast", "Cinnamon Toast"],
  ["2002-03-14-veal_cutlets", "Breaded Veal Scallopini"],
  ["2002-03-20-mushroom_pasta", "Mushroom Chicken Pasta"],
  ["2002-04-09-pasta_primavera", "Pasta Primavera"],
  ["2002-04-13-chicken", "Breaded Baked Chicken"],
  ["2002-04-15-cajun_shrimp", "Cajun Shrimp"],
  ["2002-04-30-chicken_stew", "Chicken Andouille Stew"],
  ["2002-05-19-crabcake", "Maryland Crab Cakes"],
  ["2002-05-20-eggs", "Scrambled Eggs with Spinach and Bacon"]],
 [[["2003-04-14-sandwich", "Batter-Dipped Ham and Cheese Sandwich"],
  ["2003-04-25-chicken", "Mustard Seed Chicken with Ginger Orange Sauce"],
//...

To avoid this undesirable outcome, I need to handle re-reduces when CouchDB calls my map functiona second time:

function(keys, values, rereduce) {
  if (rereduce) {
    var ret = [];
    for (var i=0; i<values.length; i++) {
      ret = ret.concat(values[i]);
    }
    return ret;
  }
  else {
    return values;
  }
}

This produces the results that I desire, a flat array of arrays:

"butter" 
[["2002-02-08-buffalo_chicken", "Buffalo Chicken Sandwich"],
 ["2002-02-13-mushroom_pasta", "Pasta with Mushroom Gruyere Sauce"],
 ["2002-02-17-sausage_pie", "Chicken Sausage Pot Pie"],
 ["2002-03-12-asparagus_omelet", "Asparagus Omelet"],
 ["2002-03-12-cinnamon_toast", "Cinnamon Toast"],
 ["2002-03-14-veal_cutlets", "Breaded Veal Scallopini"],
 ["2002-03-20-mushroom_pasta", "Mushroom Chicken Pasta"],
 ["2002-04-09-pasta_primavera", "Pasta Primavera"],
 ["2002-04-13-chicken", "Breaded Baked Chicken"],
 ["2002-04-15-cajun_shrimp", "Cajun Shrimp"],
 ["2002-04-30-chicken_stew", "Chicken Andouille Stew"],
 ["2002-05-19-crabcake", "Maryland Crab Cakes"],
 ["2002-05-20-eggs", "Scrambled Eggs with Spinach and Bacon"],
 ["2003-04-14-sandwich", "Batter-Dipped Ham and Cheese Sandwich"],
 ["2003-04-25-chicken", "Mustard Seed Chicken with Ginger Orange Sauce"],
...

I will stop there for the day. Now that I know the format of the output (and that it will work as I desire), I can drive the implementation of the ingredient index tomorrow.