Add Virtual Attribute Type RFC

rfcs/xxxx-virtual-attribute-type.md

@@ -0,0 +1,101 @@
+- Start Date: 2020-08-16
+- RFC PR: (leave this empty)
+
+# Summary
+
+It would be very useful to have "read only" attributes that are generated during "read" lifecycles (find, findOne)
+using developer-defined functions. Currently it is possible to do so for the API (by extending the result in the
+lifecycle functions). However, those attributes are not be available in any plugins (most importantly Content Manager
+and Content Type Builder), since they are not defined in the model. By adding a "virtual" attribute type, you could
+add attributes to a model without them being saved to the database, which would make them available to plugins too.
+
+Related issues: [#3131](https://github.com/strapi/strapi/issues/3131)
+
+Related PRs: [#2712](https://github.com/strapi/strapi/pull/2712), [#6433](https://github.com/strapi/strapi/pull/6433), 
+[#7221](https://github.com/strapi/strapi/pull/7221)
+
+Related discussions: [#7173](https://github.com/strapi/strapi/discussions/7173),
+[#7465](https://github.com/strapi/strapi/discussions/7465)
+
+# Example
+
+## model.settings.json
+
+```json
+{
+  ...
+  "attributes": {
+    "first_name": {
+      "type": "string",
+    },
+    "last_name": {
+      "type": "string",
+    },
+    "full_name": {
+      "type": "virtual",
+    }
+  }
+}
+```
+
+## model.js
+
+```js
+module.exports = {
+  virtualAttributes: {
+    full_name(entity) {
+      return `${entity.first_name} ${entity.last_name}`;
+    }
+  }
+}
+```
+
+# Motivation
+
+Currently it is impossible to add custom fields to admin panel that are not written to the database. As shown in this
+simple example, a user might want to show the full name as a field in the admin panel, while having two separate
+fields for first and last names. Right now the only way of doing this is adding an attribute to the model and modifying
+`create` and `update` lifecycles. This works, however it duplicates the data in the database, which is a bad solution.
+
+This would also solve issue [#3131](https://github.com/strapi/strapi/issues/3131) in a much more practical way (in my
+opinion), as this provides much much more freedom than simple formulas.
+
+# Detailed design
+
+The example shown before is pretty straightforward and should be self explanatory, but I will elaborate further. Before
+Strapi returns an entity it should run a function defined in `virtualAttributes` for each attribute of virtual type.
+Something akin to this (pseudo code):
+
+```js
+// attributes - model type attribute definitions
+// entity - model entity object with populated attributes from the database
+// virtualAttributes - object with virtual attribute functions of the model
+
+for (const key in attributes) {
+  if (attributes[key].type === 'virtual') {
+    entity[key] = virtualAttributes[key](entity);
+  }
+}
+```
+
+Things that would need to be implemented (likely missing some):
+
++ Either Database Manager plugin (ideally) or both Database Connector Bookshelf & Mongoose plugins should not save virtual attributes to the database
++ Database Manager plugin should execute `virtualAttributes` functions after `find` and `findOne` queries
++ Virtual attribute type should be implemented in Content Type Builder plugin
+
+# Tradeoffs
+
+Personally I cannot see any real tradeoffs. This seems relatively not too difficult to implement, while adding a very
+much needed feature to Strapi. It does not require any migration or interfere with existing concepts and features of
+Strapi.
+
+# Alternatives
+
+Issue [#3131](https://github.com/strapi/strapi/issues/3131) suggest having formula fields, which I imagine would have
+a similar implementation (or likely more difficult), while being much less flexible.
+
+# Unresolved questions
+
+Unsure if the `virtualAttributes` functions should run before or after the lifehooks. Probably either way works,
+however it should definitely be noted in the documentation.