🔧 Allow disabling the src/seq/v index

- Fixes #94
- Follows on from share/sharedb#657

This change is to work alongside upstream work to remove ShareDB's
usage of `getCommittedOpVersion()`.

In `sharedb-mongo` this function requires an entire extra op index,
just to handle a corner case where two `create` ops are submitted at the
same time, which should happen relatively infrequently.

This change allows consumers to opt out of individual indexes. For
example, if we're not using `getCommittedOpVersion()`, consumers can
opt out of the automatic `src`/`seq`/`v` index creation with:

```js
new ShareDbMongo(
  mongoUrl,
  {
    disableIndexCreation: {
      src_seq_v: true,
    },
  },
);
```

Note that if this index already exists, consumers will need to manually
remove it.

Previous behaviour will still work, so setting:

```js
new ShareDbMongo(
  mongoUrl,
  {
    disableIndexCreation: true,
  },
);
```

disables **all** index creation.

Author: alecgibson

index.js

@@ -384,7 +384,7 @@ ShareDbMongo.prototype.getOpCollection = function(collectionName, callback) {
     var collection = mongo.collection(name);
     // Given the potential problems with creating indexes on the fly, it might
     // be preferrable to disable automatic creation
-    if (self.disableIndexCreation) {
+    if (self.disableIndexCreation === true) {
       return callback(null, collection);
     }
     if (self.opIndexes[collectionName]) {
@@ -398,10 +398,13 @@ ShareDbMongo.prototype.getOpCollection = function(collectionName, callback) {
     // collection this won't be a problem, but this is a dangerous mechanism.
     // Perhaps we should only warn instead of creating the indexes, especially
     // when there is a lot of data in the collection.
-    collection.createIndex({d: 1, v: 1}, {background: true})
-      .then(function() {
-        return collection.createIndex({src: 1, seq: 1, v: 1}, {background: true});
-      })
+
+    var disabledIndexes = self.disableIndexCreation || {};
+    var promises = [
+      collection.createIndex({d: 1, v: 1}, {background: true}),
+      !disabledIndexes.src_seq_v && collection.createIndex({src: 1, seq: 1, v: 1}, {background: true})
+    ];
+    Promise.all(promises)
       .then(function() {
         self.opIndexes[collectionName] = true;
         callback(null, collection);

test/test_mongo.js

@@ -2,6 +2,7 @@ var expect = require('chai').expect;
 var ShareDbMongo = require('..');
 var getQuery = require('sharedb-mingo-memory/get-query');
 var async = require('async');
+var randomUUID = require('crypto').randomUUID;
 
 var mongoUrl = process.env.TEST_MONGO_URL || 'mongodb://localhost:27017/test';
 
@@ -432,6 +433,50 @@ describe('mongo db', function() {
   });
 });
 
+describe('options', function() {
+  var db;
+
+  afterEach(function(done) {
+    db.close(done);
+  });
+
+  it('disables index creation', function(done) {
+    var collection = randomUUID();
+    db = new ShareDbMongo(mongoUrl, {disableIndexCreation: true});
+    async.waterfall([
+      db.commit.bind(db, collection, 'foo', {v: 0, create: {}}, {}, null),
+      function(succeeded, next) {
+        db.getDbs(next);
+      },
+      function(mongo, mongoPoll, next) {
+        mongo.collection('o_' + collection).indexInformation().then(function(indexes) {
+          expect(indexes['d_1_v_1']).not.to.be.ok;
+          expect(indexes['src_1_seq_1_v_1']).not.to.be.ok;
+          next();
+        });
+      }
+    ], done);
+  });
+
+  it('disables just the src,seq,v index', function(done) {
+    var collection = randomUUID();
+    db = new ShareDbMongo(mongoUrl, {disableIndexCreation: {src_seq_v: true}});
+    async.waterfall([
+      db.commit.bind(db, collection, 'foo', {v: 0, create: {}}, {}, null),
+      function(succeeded, next) {
+        db.getDbs(next);
+      },
+      function(mongo, mongoPoll, next) {
+        mongo.collection('o_' + collection).indexInformation().then(function(indexes) {
+          expect(indexes['d_1_v_1']).to.be.ok;
+          expect(indexes['src_1_seq_1_v_1']).not.to.be.ok;
+          next();
+        });
+      }
+    ], done);
+  });
+});
+
 describe('mongo db connection', function() {
   describe('via url string', function() {
     beforeEach(function(done) {