Attribute alteration

[rnewman]

Datomic allows you to alter :db/cardinality, :db/isComponent, :db/noHistory, :db/index and :db/unique via :db.alter/attribute transacts. Datomic doesn't allow you to alter :db/valueType (which I think is fine) or :db/fulltext (which I think we could support with little effort).

I think we'll need to support these kinds of alterations, automatically effected by ensureSchema, in order to do very basic schema fixing/tweaking, particularly from JS code.

Note that this doesn't include actually managing schema versions and migrating data — these attribute alterations fail if the store contains incompatible data.

[rnewman]

Schema changes:

• cardinality
  • one to many: fine
  • many to one: validate
• isComponent:
  • fine
• noHistory:
  • Currently ignored, hint only -- fine.
• unique
  • nil to value: validate
  • nil to identity: validate
  • identity to value or value to identity: fine
• index
  • enable or disable the avet index.