Moved dynamic constraints in a dedicated folder

bookshop/test/dynamic-constraints/readme.md

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+## Experimental Dynamic Constraints
+
+This example demonstrates how to use dynamic constraints in a CAP application. It includes a service definition and a test setup to validate the constraints.
+
+
+### Prerequisites
+
+You've setup the [_cap/samples_](https://github.com/sap-samples/cloud-cap-samples) like so:
+
+```sh
+git clone -q https://github.com/sap-samples/cloud-cap-samples cap/samples
+cd cap/samples
+npm install
+```
+
+### Testing
+
+Test like that in `cds.repl` from _cap/samples_ root:
+
+```sh
+cds repl --run bookshop/test/dynamic-constraints
+````
+
+```javascript
+await AdminService.create ('Books', { title:'   ', author_ID:150 })
+```

bookshop/test/dynamic-constraints/server.js

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+//
+// Quick and dirty implementation for cds.validate()
+// using db-level constraints.
+//
+
+const cds = require('@sap/cds')
+cds.on('served', ()=> {
+
+  const $ = cds.validate
+  cds.validate = function (entity, key, ...columns) {
+
+    if (entity?.ref) entity = { // quick and dirty
+      name: entity.ref[0],
+      constraints: { // even quicker and dirtier
+        name: entity.ref[0] +'.constraints',
+        keys: {ID:1}
+      }
+    }
+    else if (!entity.is_entity) return // we skip all standard validations for the experiments
+    else if (!entity.is_entity) return $(...arguments) // eslint-disable-line no-dupe-else-if
+
+    if (entity.constraints) entity = entity.constraints
+    if (!key) return key => cds.validate(entity,key)
+    if (key.results) key = key.results[0].lastInsertRowid // quick and dirty
+    if (key.ID) key = key.ID // quick and dirty
+
+    return SELECT.one.from (entity, key, columns.length && columns) .then (checks => {
+      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}`
+    })
+  }
+
+  const { AdminService } = cds.services
+  AdminService.after (['CREATE','UPDATE'], (result,req) =>  cds.validate (req.subject, result))
+})
+
+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/test/dynamic-constraints/srv/admin-service.cds

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+using { AdminService } from '../../../srv/admin-service';
+annotate AdminService with @requires: false;
+extend AdminService.Authors with columns {
+  null as books // to simulate the exclusion of books
+}

bookshop/test/dynamic-constraints/srv/field-control.cds

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+namespace sap.capire.bookshop;
+using from './admin-service';
+
+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/test/dynamic-constraints/srv/validation.cds

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+using { AdminService, sap.capire.bookshop as my } from './admin-service';
+
+extend service AdminService with {
+
+  // entity Books.drafts as projection on AdminService.Books;
+  // @cds.api.ignore view Books.drafts.constraints as select from AdminService.Books.drafts mixin {
+  //   before: Association to my.Books on before.ID = $self.ID;
+  //   base: Association to my.Books on base.ID = $self.ID;
+  // } into { ID, // FIXME: compiler should resolve Books without AdminService prefix
+  //   case
+  //     when title is null  then 'is missing'
+  //     when trim(title)='' then 'must not be empty'
+  //   end as title,
+  //   ...
+  // }
+
+  /**
+   * Validation constraints for Books
+   */
+  @cds.api.ignore view Books.constraints as select from AdminService.Books mixin {
+    base: Association to my.Books on base.ID = $self.ID;
+  } into { ID, // FIXME: compiler should resolve Books without AdminService prefix
+
+    // two-step mandatory check
+    case
+      when title is null  then 'is missing'
+      when trim(title)='' then 'must not be empty'
+    end as title,
+    // the above is equivalent to:
+    // title is null ? 'is missing' : trim(title)='' ? 'must not be empty' :
+
+    // 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
+    author.ID is null ? 'is missing' : // FIXME: 1) // TODO: 2)
+    not exists author ? 'Author does not exist: ' || author.ID :
+    count(base.author.books.ID) -1 > 1 ? author.name || ' already wrote too many books' : // TODO: 3)
+    null as author,
+
+  } group by ID;
+
+  // 1) FIXME: expected author.ID to refer to foreign key,
+  // apparently that is not the case -> move one line up
+  // and run test to see the erroneous impact.
+
+  // 2) TODO: we should allow to write author is null instead of author.ID is null
+
+  // 3) TODO: we should support count(author.books)
+
+
+  /**
+   * Validation constraints for Authors
+   */
+  view Authors.constraints as select from AdminService.Authors { ID, // FIXME: compiler should resolve Authors without AdminService prefix
+
+    // two-step mandatory check
+    name = null ? 'is missing' : trim(name)='' ? 'must not be empty' :
+    null as name,
+
+    // constraint related to two fields
+    dateOfDeath > dateOfBirth ? 'we must be born before we die' : null as _born_before_death,
+    $self._born_before_death as dateOfBirth,
+    $self._born_before_death as dateOfDeath,
+
+  }
+}