Add model api docs

Author: Pasquale Latrofa

content/en/docs/apidocs-mxsdk/apidocs/studio-pro/extensibility-api/web/web-extensions-howtos/model-api.md

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+---
+title: "Model Api"
+url: /apidocs-mxsdk/apidocs/extensibility-api/web/model-api/
+weight: 6
+---
+
+## Introduction 
+
+This how-to provides guidance on using the Model Access API:
+
+* [Using the Model Access API](#using-api)
+* [Reading the units info and loading units](#units-info-load)
+* [Reading the unit content](#read)
+* [Modifying the unit content](#modify)
+
+## Using the Model Access API {#using-api}
+
+The Model Access API allow access to the Mendix model.
+
+The model is split in several components exposed via `studioPro.app.model` object. Currently supported components are:
+* BuildinBlocks
+* DomainModels
+* Enumerations
+* Pages
+* Snippets
+
+```ts
+const { pages, domainModels } = studioPro.app.model;
+```
+
+## Reading the units info and loading units {#units-info-load}
+
+An element is part of a Mendix model and all elements together form the logic of the model. Elements may contain other elements. An element always has a container element, which is its parent. The root of an element tree is always a unit.
+
+Each component exposes the units info of the units it is responsible for. The full unit content can be accessed only after loading the unit.
+
+The unit info contains the the following fields:
+| Name | Description | 
+| --- | --- |
+| `$ID` | The unique id of the unit |
+| `$Type` | The type of the unit |
+| `moduleName` | (Optional) The name of the module containing the unit |
+| `name` | (Optional) The name of the unit |
+
+All the units managed by the DomainModels component can be retrieved by:
+
+```ts
+const unitsInfo = await domainModels.getUnitsInfo()
+```
+
+Units can be loaded by suppling a function to `loadAll` to execute for each unit. It should return a truthy value to load the unit.
+
+The followind snippet loads the DomainModel for the module named `MyFirstModule`:
+
+{{% alert color="warning" %}}
+Loading units is an expensive process. Only load the minimun number of units you need when you need them.
+{{% /alert %}}
+
+```ts
+const [domainModel] = await domainModels.loadAll((info) => info.moduleName === 'MyFirstModule');
+```
+
+## Reading the unit content {#read}
+
+Elements contained inside units can be accessed using the `get<ElementName>` helper methods:
+
+The following snippet will get the Entity named `MyEntity` from the previously loaded DomainModel unit:
+
+```ts
+const entity = domainModel.getEntity("MyEntity");
+```
+
+## Modifying the unit content {#modify}
+
+The Mendix model can be modified by leveraing the `add<ElementName>` helper methods:
+
+The following snippet will create a new Entity inside the previously loaded DomainModel unit:
+
+```ts
+const newEntity = await domainModel.addEntity({ name: "NewEntity", attributes: [{ name: "MyAttribute", type: "AutoNumber" }]})
+
+await domainModels.save(domainModel);
+```
+