[template-engine-preview-5] Transforms returning objects without _id

[mystor]

If you return an object without an _id property from a collection transform, it produces a very cryptic error if a cursor for the collection is later used in a template.

// javascript
Things = new Meteor.Collection('things', {
  transform: function(doc) {
    return { name: doc.name };
  }
});

Template.tpl_things.things = function() {
  return Things.find();
}

<!-- html -->
<template name="tpl_things">
  <ul>
    {{#each things}}
      <li>{{name}}</li>
    {{/each}}
  </ul>
</template>

I understand that having an _id property is useful for objects returned by a transform, but it would be nice to have a check to ensure that an _id field is returned if future code will rely on its presence.

This code runs without issue on the current release of meteor (0.6.6.3).

[avital]

Thanks @mystor. Agreed that we should supply a better error in case a transform function returns documents with no _id property.

I'd just like to ask: Requiring an _id property is a change from 0.6.6.3. Given your experience with transforms, do you think there are cases when forcing the developer to supply an _id will be difficult? Or would it always just be passing in the _id from the original document being transformed?

[avital]

I chatted with @sixolet and @glasser about this and we're probably going to make the following changes to transform:

1. Only allow objects to be returned
2. After the transform function is called, an _id property is added to the returned object. The _id is taken from the original untransformed document. (If transform already returns an object with an _id field, verify that it is the same as the original document's _id)

This will resolve the problem without requiring users to specify _id manually.

[mystor]

I think that this is a good option. If the _id property is required, then it makes sense to force the transform function to return objects, and to manually set the _id property to prevent user error.

The only situation which I can think of for wanting to have something without an _id property would be a document like the following:

{
  "_id": "asdfgh",
  "text": "abc123"
}

Where the transform would just return the string "abc123", to remove the requirement for a property access (which could fail if Collection.findOne() returned undefined). I feel that preventing that use case is probably good, as it encourages better practices and ensures that people don't build their projects in an unmaintainable way. A transform without an _id property means that removing and updating the object will be much more difficult.

[avital]

Fixed on fe661ee