feat: PoC for dynamic constraints using standard cds views

bookshop/server.js

@@ -0,0 +1,26 @@
+//
+//  Quick and dirty implementation for cds.validate() using db-level constraints
+//
+
+const cds = require('@sap/cds')
+cds.on('served', ()=> {
+
+  const $ = cds.validate; cds.validate = async function (entity, key, ...columns) {
+    if (!entity.is_entity) return $(...arguments)
+    if (entity.constraints) entity = entity.constraints
+    if (key?.results) key = key.results[0].lastInsertRowid
+    const checks = await SELECT.one.from (entity, key, columns.length && columns)
+    const failed = {}; for (let c in checks) {
+      if (c in entity.keys) continue
+      if (c[0] == '_') continue
+      if (checks[c]) failed[c] = checks[c]
+    }
+    if (Object.keys(failed).length) throw cds.error `Invalid input: ${failed}`
+  }
+
+})
+
+Object.defineProperties (cds.entity.prototype, {
+  constraints: { get() { return cds.model.definitions[this.name+'.constraints'] }},
+  fields: { get() { return cds.model.definitions[this.name+'.field.control'] }},
+})

bookshop/srv/x-field-control.cds

@@ -0,0 +1,10 @@
+namespace sap.capire.bookshop;
+using from '../db/schema';
+
+view Books.field.control as select from Books { ID,
+  genre.name == 'Drama' ? 'readonly' :
+  null as price
+}
+extend Books with {
+  fc : Association to Books.field.control on fc.ID = $self.ID
+}

bookshop/srv/x-validation.cds

@@ -0,0 +1,50 @@
+namespace sap.capire.bookshop;
+using from '../db/schema';
+
+
+/**
+ * Validation constraints for Books
+ */
+view Books.constraints as select from Books { ID,
+
+  // two-step mandatory check
+  title = null ? 'is missing' : trim(title)='' ? 'must not be empty' :
+  null as title,
+
+  // range check
+  stock < 0 ? 'must not be negative' :
+  null as stock,
+
+  // range check
+  price < 0 ? 'must not be negative' :
+  null as price,
+
+  // assert target check
+  genre.ID is not null and not exists genre ? 'does not exist' :
+  null as genre,
+
+  // multiple constraints: mandatory + assert target + special
+  not exists author ? 'Author does not exist: ' || author.ID :
+  author.ID is null ? 'is missing' : // FIXME: expected author.ID to refer to foreign key, apparently that is not the case -> move one line up to see
+  count(author.books.ID) -1 > 1 ? author.name || ' already wrote multiple books, please choose another author' : // TODO: we should support count(author.books)
+  null as author,
+
+} group by ID;
+
+
+
+/**
+ * Validation constraints for Authors
+ */
+view Authors.constraints as select from Authors { ID,
+
+  // two-step mandatory check
+  name = null ? 'is missing' : trim(name)='' ? 'must not be empty' :
+  null as name,
+
+  // constraint related to two fields
+  dateOfDeath > dateOfBirth ? 'date of birth must be before date of death' : null as _born_before_death,
+  $self._born_before_death as dateOfBirth,
+  $self._born_before_death as dateOfDeath,
+
+}