Option to ignore item validation errors when value is unchanged

[OldSneerJaw]

Feature Request

Description

Occasionally, when updating document definitions, it is necessary to introduce a breaking change to the schema. However, if there are already documents in the database that conform to the old schema but not the new schema (e.g. a string's regexPattern or an integer's minimumValue have changed), this can require a full, complicated data migration to the new format that may result in revision conflicts if any of the documents in question also happen to have been modified by an offline client. Sometimes this is unavoidable, but in other cases it is acceptable to "grandfather" the old value in for existing documents (e.g. a product ID format that is no longer used for new products, but is still valid for old, obsolete products).

This feature would introduce a new universal item validation constraint (e.g. skipValidationWhenValueUnchanged) that suppresses validation errors for that property/element when replacing a document revision where the corresponding property/element has the same semantic value. Also introduce a constraint that requires strict equality (e.g. skipValidationWhenValueUnchangedStrict).

Examples

Consider the following document definition:

myDoc: {
  typeFilter: simpleTypeFilter,
  authorizedRoles: { write: 'write' },
  propertyValidators: {
    code: {
      type: 'string',
      regexPattern: /^[a-z]{4}-\d{3}$/,
      skipValidationWhenValueUnchanged: true
    },
    quantity: {
      type: 'integer',
      required: true,
      minimumValue: 0
    }
  }
}

The code property must be four lowercase letters, followed by a dash and three digits. However, if the previous version of the code property's regexPattern expected only two digits at the end, then values from existing documents that match that regular expression would be invalid if not for the skipValidationWhenValueUnchanged option.

Therefore with a previous document body of { "code": "abcd-12", "quantity": 5 }, the following new document revision would be considered valid even though it validates the updated regexPattern for code: { "code": "abcd-12", "quantity": 4 }

[dkichler]

Addressed by #35 and #36