Creating the Meta-model ====================== At this point you should have a basic understanding of the concepts of meta-modeling in webgme and be somewhat familiar with the GUI of webgme. In this section we will build up a meta-model for the electrical circuits domain. Which types do we need? -------------------------- The first thing to layout when constructing a meta-model is what types our domain should include. Which these are does not only depend on the domain, but also the analysis tools being targeted. Since our target is **Modelica** and more specifically the Analog Electrical library as part of the **Modelica Standard Library**, we should take that into consideration. .. figure:: electrical_analog_modelica.png :align: center :scale: 80 % Electrical Analog components in Modelica This library contains a range of electrical components with a various number of pins. Looking at one of the examples we see that the pins are connected via electrical connections. .. figure:: cauer_low_pass_modelica.png :align: center :scale: 80 % A Cauer Low Pass Analog Circuit in Modelica With or without Modelica a very natural breakdown of this domain is include the types: :code:`Component`, :code:`Pin`, and :code:`Connection`. Additionally, since our goal is to build electrical circuits, we also need a :code:`Circuit` type. Later we will add sub-types of components corresponding to components such as ``Resistor``, ``Ground``, ``Capacitor``, ``Inductor``, etc. The video below shows how you can add these types as meta-nodes starting from your empty project. `Click here to open the video in a browser. `_. .. raw:: html

| Containment ---------------- Now let's model where these concepts can be added in the containment-hierarchy. A ``Circuit`` should be able to contain ``Components`` wired together by ``Connections``. The way connections are constructed in webgme requires us to add a containment rule for the ``Connection`` w.r.t. the ``Circuit``. Next section illustrates how we can make the ``Connection`` in to an actual connection (an edge on the drawing canvas). Inside the ``Component`` the ``Pins`` determine where the ``Connections`` connect the ``Components`` together. The video below shows how to add these containment rules to our meta-model using the Meta Editor. `Click here to open the video in a browser. `_. .. raw:: html

| Sub-types of Components -------------------- So far our meta-model only contains a generic ``Component`` for representing electrical components, but we need a way to represent specific electrical components such as ``Resistor``, ``Ground``, etc. There are multiple ways we can achieve this by extending the meta-model. One apporach is to add a meta-type for each type of electrical component and add the related Modelica parameters as attributes to these respectively. Another approach is to create the different types of electrical components outside of the meta-model and treat the Modelica parameters as separate child nodes of the ``Components``. This approach allows for creation of new types without modifying the meta-model itself, but also makes the modeling a bit more cumbersome using the default visualization. (Visualizing and modifying the parameters of a component could no longer be done from a single node.) For the sake of simplicity we will take the first approach and limit our domain to the five ``Components`` below (we will also leave out the heat transfer portion in the domain). The associated Modelica parameters can be extracted from the **Modelica Standard Library** using a Modelica tool, such as `OpenModelica `_. For each component we need to indicate its unique path or identifier within the **MSL**, this will be captured by the read-only attribute ``ModelicaURI``. In order to map directly to Modelica we name the ports and the other parameters the same way they're named in **MSL**. Resistor Attributes - ``ModelicaURI`` - ``Modelica.Electrical.Analog.Basic.Resistor`` - ``R`` - The resistance of the resistor in Ohm. A float greater or equal to ``0`` with a default value of ``1``. Ports Two ``Pins``, ``p`` and ``n``. Ground Modelica requires each electrical system (``Circuit`` in our case) to contain a ground component in order to make the system solvable. Attributes - ``ModelicaURI`` - ``Modelica.Electrical.Analog.Basic.Ground`` Ports One ``Pin``, named `p`. Inductor Attributes - ``ModelicaURI`` - ``Modelica.Electrical.Analog.Basic.Inductor`` - ``L`` - The inductance of the inductor in Henry. A float greater or equal to ``0`` with a default value of ``1``. Ports Two ``Pins``, ``p`` and ``n``. Capacitor Attributes - ``ModelicaURI`` - ``Modelica.Electrical.Analog.Basic.Capacitor`` - ``C`` - The capacitance of the capacitor in Farad. A float greater or equal to ``0`` with a default value of ``1``. Ports Two ``Pins``, ``p`` and ``n``. StepVoltage Attributes * ``ModelicaURI`` - ``Modelica.Electrical.Analog.Sources.StepVoltage`` * ``V`` - The voltage of the source in Volt. A float with a default value of ``1``. * ``startTime`` - Time offset (when the voltage goes from 0 to ``V``) in seconds. A float with a default value of ``1``. Ports Two ``Pins`` ``p`` and ``n``. With the approach taken the ``Component`` meta-type itself will not have any interpretation w.r.t. our domain and will only act as an `abstract` type that cannot be instantiated. In the video an additional `abstract` base type, ``TwoPinComponent``, that defines two ``Pins``, ``p`` and ``n`` is also added. In general this approach is not only more elegant and convenient, but also more efficient since the raw data for the two pins can be shared and requires less data to be loaded from the server. `Click here to open the video in a browser. `_. .. raw:: html

| Connections and Ports -------------------- In order to create connections between ``Components`` or rather between the ``Pins`` of the ``Components`` we need to make our ``Connection`` into a connection like object. In webgme's meta-model there is no first order concept of a connection, instead such can be constructed using reserved named pointers; ``src`` and ``dst``. The target of each will be the source and destination of the ``Connection`` respectively. For more details on connections revisit the videos in the **Meta-modeling Concepts** section. Just like with connections, there is no first order concept of a port in webgme either. Connection sources and destinations are only constrained by the valid pointer ends defined in meta-model and can crosscut over the containment hierarchy. To make modeling more comprehensible, it is often useful to be able to visually propagate port like children up to boundary of the parent node. The way this is solved in webgme is through the meta-property (implemented as a registry) ``isPort``. Note that the usage of this property only takes effect if the decorator (the UI component responsible for drawing the box on the canvas) implements logic using this property - this is the case for both the default Model- and SVG-Decorators. `Click here to open the video in a browser. `_. .. raw:: html

| At this point we have a complete meta-model and a small example ``Circuit``.