fix: do not apply default values on data from database

Before this change, when a property was configured with a default value
at LoopBack side and the database was returned a record with a missing
value for such property, then we would supply use the configured
default.

This behavior is problematic for reasons explained in #1692.

In this commit, we are introducing a new model-level setting called
`applyDefaultsOnReads`, which is enabled by default for backwards
compatibility.

When this setting is set to `false`, operations like `find` and
`findOrCreate` will NOT apply default property values on data returned
by the database (connector).

Please note that most of the other CRUD methods did not apply default
values on database data as long as the connector provided native
implementation of the operation, that aspect is not changing.

lib/dao.js

@@ -1109,8 +1109,15 @@ DataAccessObject.findOrCreate = function findOrCreate(query, data, options, cb)
         let obj;
 
         if (data) {
-          obj = new Model(data, {fields: query.fields, applySetters: false,
-            persisted: true});
+          const ctorOpts = {
+            fields: query.fields,
+            applySetters: false,
+            persisted: true,
+          };
+          if (Model.settings.applyDefaultsOnReads === false) {
+            ctorOpts.applyDefaultValues = false;
+          }
+          obj = new Model(data, ctorOpts);
         }
 
         if (created) {
@@ -1632,6 +1639,9 @@ DataAccessObject.find = function find(query, options, cb) {
             applySetters: false,
             persisted: true,
           };
+          if (Model.settings.applyDefaultsOnReads === false) {
+            ctorOpts.applyDefaultValues = false;
+          }
           let obj;
           try {
             obj = new Model(data, ctorOpts);

test/basic-querying.test.js

@@ -922,6 +922,60 @@ describe('basic-querying', function() {
           });
         });
     });
+
+    it('applies default values by default', async () => {
+      // Backwards compatibility, see
+      // https://github.com/strongloop/loopback-datasource-juggler/issues/1692
+
+      // Initially, all Products were always active, no property was needed
+      const Product = db.define('Product', {name: String});
+      await db.automigrate('Product');
+      const created = await Product.create({name: 'Pen'});
+
+      // Later on, we decide to introduce `active` property
+      Product.defineProperty('active', {
+        type: Boolean,
+        default: false,
+      });
+
+      // And query existing data
+      const found = await Product.findOne();
+      found.toObject().should.eql({
+        id: created.id,
+        name: 'Pen',
+        // Backwards-compatibility
+        // When Pen does not have "active" flag set, we change it to default
+        active: false,
+      });
+    });
+
+    it('preserves empty values from the database when "applyDefaultsOnReads" is false', async () => {
+      // https://github.com/strongloop/loopback-datasource-juggler/issues/1692
+
+      // Initially, all Products were always active, no property was needed
+      const Product = db.define(
+        'Product',
+        {name: String},
+        {applyDefaultsOnReads: false},
+      );
+
+      await db.automigrate('Product');
+      const created = await Product.create({name: 'Pen'});
+
+      // Later on, we decide to introduce `active` property
+      Product.defineProperty('active', {
+        type: Boolean,
+        default: false,
+      });
+
+      // And query existing data
+      const found = await Product.findOne();
+      found.toObject().should.eql({
+        id: created.id,
+        name: 'Pen',
+        active: undefined,
+      });
+    });
   });
 
   describe('count', function() {

test/manipulation.test.js

@@ -1619,6 +1619,60 @@ describe('manipulation', function() {
         })
         .catch(done);
     });
+
+    it('applies default values on returned data', async () => {
+      // Backwards compatibility, see
+      // https://github.com/strongloop/loopback-datasource-juggler/issues/1692
+
+      // Initially, all Products were always active, no property was needed
+      const Product = db.define('Product', {name: String});
+      await db.automigrate('Product');
+      const created = await Product.create({name: 'Pen'});
+
+      // Later on, we decide to introduce `active` property
+      Product.defineProperty('active', {
+        type: Boolean,
+        default: false,
+      });
+
+      // and findOrCreate an existing record
+      const [found] = await Product.findOrCreate({id: created.id}, {name: 'updated'});
+      found.toObject().should.eql({
+        id: created.id,
+        name: 'Pen',
+        // Backwards-compatibility
+        // When Pen does not have "active" flag set, we change it to default
+        active: false,
+      });
+    });
+
+    it('preserves empty values from the database when "applyDefaultsOnReads" is false', async () => {
+      // https://github.com/strongloop/loopback-datasource-juggler/issues/1692
+
+      // Initially, all Products were always active, no property was needed
+      const Product = db.define(
+        'Product',
+        {name: String},
+        {applyDefaultsOnReads: false},
+      );
+
+      await db.automigrate('Product');
+      const created = await Product.create({name: 'Pen'});
+
+      // Later on, we decide to introduce `active` property
+      Product.defineProperty('active', {
+        type: Boolean,
+        default: false,
+      });
+
+      // And findOrCreate an existing record
+      const [found] = await Product.findOrCreate({id: created.id}, {name: 'updated'});
+      found.toObject().should.eql({
+        id: created.id,
+        name: 'Pen',
+        active: undefined,
+      });
+    });
   });
 
   describe('destroy', function() {

types/model.d.ts

@@ -76,6 +76,13 @@ export interface ModelProperties {
 export interface ModelSettings extends AnyObject {
   strict?: boolean;
   forceId?: boolean;
+
+  /*
+   * Apply default property values to data fetched from database.
+   * Enabled by default.
+   * TODO(semver-major): disable by default
+   */
+  applyDefaultsOnReads?: boolean;
 }
 
 /**