[validation] Run custom validators on undefined values unless rule is…

… set to optional

packages/@sanity/validation/src/Rule.js

@@ -6,6 +6,15 @@ const createUriRegex = require('./util/createUriRegex')
 const knownTypes = ['Object', 'String', 'Number', 'Boolean', 'Array', 'Date']
 const isExclusive = ['type', 'uri', 'email']
 
+const mergeRequired = (prev, next) => {
+  let isRequired = prev._required || next._required
+  if (!isRequired) {
+    isRequired = prev._required === false || next._required === false ? false : undefined
+  }
+
+  return isRequired
+}
+
 class Rule {
   static FIELD_REF = Symbol('FIELD_REF')
   static array = () => new Rule().type('Array')
@@ -46,13 +55,13 @@ class Rule {
     this._type = this._type || null
     this._rules = (this._rules || []).filter(rule => rule.flag === 'type')
     this._message = null
-    this._required = false
+    this._required = undefined
     this._level = 'error'
     return this
   }
 
   isRequired() {
-    return this._required
+    return this._required === true
   }
 
   clone() {
@@ -95,7 +104,7 @@ class Rule {
     const newRule = this.cloneWithRules(rule._rules)
     newRule._type = this._type || rule._type
     newRule._message = this._message || rule._message
-    newRule._required = this._required || rule._required
+    newRule._required = mergeRequired(this, rule)
     newRule._level = this._level === 'error' ? rule._level : this._level
     return newRule
   }

packages/@sanity/validation/src/validate.js

@@ -13,15 +13,21 @@ const typeValidators = {
 }
 
 module.exports = (rule, value, options = {}) => {
-  const type = rule._type
-  const validators = typeValidators[type] || genericValidator
+  let rules = rule._rules
 
-  // Short-circuit on optional, empty fields
-  if (!rule._required && (value === null || typeof value === 'undefined')) {
+  const valueIsUndefined = value === null || typeof value === 'undefined'
+  if (typeof rule._required === 'undefined' && valueIsUndefined) {
+    // Run all _custom_ functions if the rule is not set to required or optional
+    rules = rules.filter(curr => curr.flag === 'custom')
+  } else if (!rule._required && valueIsUndefined) {
+    // Short-circuit on optional, empty fields
     return Promise.resolve([])
   }
 
-  const tasks = rule._rules.map(createValidationTask)
+  const type = rule._type
+  const validators = typeValidators[type] || genericValidator
+
+  const tasks = rules.map(createValidationTask)
   return Promise.all(tasks)
     .then(results => results.filter(Boolean))
     .then(flatten)