feat: PoC for dynamic constraints using standard cds views

bookshop/srv/x-field-control.cds

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+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

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+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,
+
+}