Comment 5 for bug 1800872

Looking at the code, the description of the modelEntityRefs document is:
type modelEntityRefsDoc struct {
 UUID string `bson:"_id"`

 // Machines contains the names of the top-level machines in the model.
 Machines []string `bson:"machines"`

 // Applications contains the names of the applications in the model.
 Applications []string `bson:"applications"`

 // Volumes contains the IDs of the volumes in the model.
 Volumes []string `bson:"volumes"`

 // Filesystems contains the IDs of the filesystems in the model.
 Filesystems []string `bson:"filesystems"`
}

However, there is no reference in 'upgrades/' to 'volumes' or 'filesystems' that would be adding those values to the documents in the database. Nor is there any reference to 'modelEntityRefs' in upgrades.

And I can confirm that "{$size: 0}" does *not* match non-existent members.

So I think we just have models that have existed before Andrew's change (6e00efc) in 2016-12 that introduced the fields. (It appears to have been introduced in Juju 2.2 so the model would have been created w juju 2.1).

It seems we have a couple possible fixes:

a) Introduce an upgrade step that adds the arrays
b) Change the cleanup code from:
   {"machines", bson.D{{"$size", 0}}},
   {"applications", bson.D{{"$size", 0}}},
   {"volumes", bson.D{{"$size", 0}}},
   {"filesystems", bson.D{{"$size", 0}}},

to something like:
   {"machines", bson.D{{"$size", 0}}},
   {"applications", bson.D{{"$size", 0}}},
   {"$or", bson.D{{"volumes", bson.D{{"$size", 0}}}, bson.D{{"volumes", bson.D{{"$exists", false}}}},
   {"$or", bson.D{{"filesystems", bson.D{{"$size", 0}}}, bson.D{{"filesystems", bson.D{{"$exists", false}}}},

or possibly:
   {"machines", bson.D{{"$size", 0}}},
   {"applications", bson.D{{"$size", 0}}},
   {"$or", bson.D{{bson.M{"volumes": bson.M{"$size": 0}}},
...
Forgive the formatting, as I'm just typing this here without trying to be precise.
The DB query would look something like:
db.modelEntityRefs.find({ $or: [ { "volumes": { $exists: false}}, { "volumes": {$size: 0}}]})
an object with an array of objects is clumsy to type in bson.D objects.

An upgrade step means we don't have to worry about them being different in future versions, a change to the query makes it easier to be compatible. (It feels a little odd to add an upgrade step in 2.5.2 for a data change in 2.2.0).