Setting "unique: false" in a schema seems to automatically imply "index: true"

[sean-daley]

Do you want to request a feature or report a bug?
Report a potential bug

What is the current behavior?
If I set "unique: false" on an attribute in a schema, the collection that gets created is automatically getting an index for that attribute. Even when I do not set "index: true"

If the current behavior is a bug, please provide the steps to reproduce.

const mongoose = require("mongoose");

const { Schema } = mongoose;

const userSchema = new Schema({
  email: {
    type: Schema.Types.String,
    required: true
  },
  username: {
    type: Schema.Types.String,
    unique: false,
    required: true
  },
});

const User = mongoose.model("User", userSchema);

const main = async () => {
   const client = await mongoose.connect('mongodb://localhost:27017/test', {
  connectTimeoutMS: 3000
});
  const user = new User({
    username: "username",
    email: "user@example.com",
  });

  const dbUser = await user.save();
  console.log(dbUser.toObject());
  client.disconnect();
}

main();

If you do a db.users.getIndices() after doing this you will see a regular index on username.
If you change the schema by either removing unique: false or adding index: false then the index goes away.

What is the expected behavior?
It would seem like just having "unique: false" without the other attributes should not generate an index automatically.

Our primary use for doing this is we have helper methods we use to generate all of our schema attributes. These methods take true/false values for things like trim, unique, lowercase, etc. If we have to, we can try and go and fix our schemas to avoid setting unique at all instead of putting unique: false but it seems like this might just be a bug?

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
Node: v14.17.2
Mongoose: 6.0.6
Mongodb: 4.2.13

Note, this has been happening for a long while now (so with older versions of mongoose 5.x as well). So this behavior isn't new.

[IslandRhythms]

https://mongoosejs.com/docs/api.html#schematypeoptions_SchemaTypeOptions-unique

[sean-daley]

https://mongoosejs.com/docs/api.html#schematypeoptions_SchemaTypeOptions-unique

I've read that documentation but it says "when truthy it will build a unique index on ...". I'm not passing in a truthy value though.

[IslandRhythms]

My bad I misread your code

[IslandRhythms]

const mongoose = require("mongoose");

const { Schema } = mongoose;

const userSchema = new Schema({
  email: {
    type: Schema.Types.String,
    required: true
  },
  username: {
    type: Schema.Types.String,
    unique: false,
    required: true
  },
});

const User = mongoose.model("User", userSchema);

const main = async () => {
   const client = await mongoose.connect('mongodb://localhost:27017/test', {
  connectTimeoutMS: 3000
});
await mongoose.connection.dropDatabase();
  const user = new User({
    username: "username",
    email: "user@example.com",
  });

  const dbUser = await user.save();
  console.log(dbUser.toObject());
  console.log(await User.listIndexes())
  client.disconnect();
}

main();

[vkarpov15]

Fix will be in v6.0.8.

For posterity, this issue is closely related to #7620