# EXPRESS ## Introduction Let's take a look a very basic **EXPRESS** data model. ```javascript SCHEMA Family; ENTITY Person name: STRING; END_ENTITY; ``` We have defined a custom schema called **`Family`**. We have a **`Person`** class within this schema. The person has one **attribute**: `name`. The name is represented as a string. | | | | --------------- | ------ | | Schema | Family | | Class Name | Person | | Class Attribute | Name | {% hint style="info" %} If you are familiar with object oriented programming, then this should feel very straight forward.\ \ If not, check out: * [JavaScript Classes](https://www.w3schools.com/js/js_class_intro.asp), or * [Python Classes](https://www.w3schools.com/python/python_classes.asp) {% endhint %} ## Window Example Let's take a look at our previous example, the window: ```python ENTITY IfcWindow SUBTYPE OF (IfcBuildingElement); OverallHeight : OPTIONAL IfcPositiveLengthMeasure; OverallWidth : OPTIONAL IfcPositiveLengthMeasure; END_ENTITY; ``` We can see that we each "object" (window, wall, door, roof, etc.) is an **`ENTITY`**. Again, notice how we use the following format to specify an "object": ```javascript ENTITY { NAME } // content goes here END_ENTITY; ``` Here's a list of some common building elements. These are defined in **IFC2x Edition 3 Technical Corrigendum 1** schema, `SCHEMA IFC2X3;`. Again, notice how each building element ("object") is its own **`ENTITY`.** ![](https://1338860669-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MSCCfuWwSmlrlrlRZ9a%2F-MSCM34TpyUsjYpuPoQu%2F-MSCMARduI7L85Bf-Aoy%2Fimage.png?alt=media\&token=2faa8e60-178d-4e0f-b8b5-7f3e0ea55bbc) Let's go back to our window. This is a **`Window`** class defined using the EXPRESS data model format. ```python SCHEMA IFC2X3; ENTITY IfcWindow SUBTYPE OF (IfcBuildingElement); OverallHeight : OPTIONAL IfcPositiveLengthMeasure; OverallWidth : OPTIONAL IfcPositiveLengthMeasure; END_ENTITY; ``` Take a look at line 5 and 6. We can see that we have two **attributes**: 1. Overall Height 2. Overall Width ### Missing Attributes? Let's continue with our Window example. We saw that the windows has two attrbitues. However, according to the [documentation](https://standards.buildingsmart.org/IFC/RELEASE/IFC2x3/FINAL/HTML/ifcsharedbldgelements/lexical/ifcwindow.htm), we're missing some attributes: ![](https://1338860669-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MSCCfuWwSmlrlrlRZ9a%2F-MSCM34TpyUsjYpuPoQu%2F-MSCMDLZ_xIYU9ruV3mc%2Fimage.png?alt=media\&token=406da914-b97d-4040-9fa8-c89eee268312) We can verify this by looking at the **STEP-file**. This has clearly more than two attributes: ```javascript #13067= IFCWINDOW('2cXV28XOjE6f6irgi0CO$D',#42,'M_Fixed:0915 x 1830mm:353953',$,'M_Fixed:0915 x 1830mm',#35337,#13061,'353953',1830.,914.999999999999); ``` We have 10 different attributes. Notice that we don't understand what any of these attributes mean. | Attribute | Value | | --------- | ----------------------------- | | #1 | 2cXV28XOjE6f6irgi0CO$D | | #2 | #42 | | #3 | M\_Fixed:0915 x 1830mm:353953 | | #4 | $ | | #5 | M\_Fixed:0915 x 1830mm:353953 | | #6 | #35337 | | #7 | #13061 | | #8 | 353953 | | #9 | 1830. | | #10 | 914.999999999999999 | ### Let's dive deep into EXPRESS How does this work? How can we have more than two attributes? ```python SCHEMA IFC2X3; ENTITY IfcWindow SUBTYPE OF (IfcBuildingElement); OverallHeight : OPTIONAL IfcPositiveLengthMeasure; OverallWidth : OPTIONAL IfcPositiveLengthMeasure; END_ENTITY; ``` Well, the answer lies within the EXPRESS definition. Take a close look at the 4th line above. We can see that **`IfcWindow`** is an **subtype** of an **`IfcBuildingElement`.** Here's the EXPRESS data model for**`IfcBuildingElement`:** ```python SCHEMA IFC2X3; ENTITY IfcBuildingElement ABSTRACT SUPERTYPE OF (ONEOF (IfcBeam ,IfcBuildingElementComponent ,IfcBuildingElementProxy ,IfcColumn ,IfcCovering ,IfcCurtainWall ,IfcDoor ,IfcFooting ,IfcMember ,IfcPile ,IfcPlate ,IfcRailing ,IfcRamp ,IfcRampFlight ,IfcRoof ,IfcSlab ,IfcStair ,IfcStairFlight ,IfcWall ,IfcWindow)) SUBTYPE OF (IfcElement); END_ENTITY; ``` Notice how the **`IfcBuildingElement`** itself is a subtype of **`IfcElement`** (see line 25 above). Let's continue down the rabbit hole... ``` ENTITY IfcElement ABSTRACT SUPERTYPE OF (ONEOF (IfcBuildingElement ,IfcDistributionElement ,IfcElectricalElement ,IfcElementAssembly ,IfcElementComponent ,IfcEquipmentElement ,IfcFeatureElement ,IfcFurnishingElement ,IfcTransportElement ,IfcVirtualElement)) SUBTYPE OF (IfcProduct); Tag : OPTIONAL IfcIdentifier; INVERSE HasStructuralMember : SET [0:?] OF IfcRelConnectsStructuralElement FOR RelatingElement; FillsVoids : SET [0:1] OF IfcRelFillsElement FOR RelatedBuildingElement; ConnectedTo : SET [0:?] OF IfcRelConnectsElements FOR RelatingElement; HasCoverings : SET [0:?] OF IfcRelCoversBldgElements FOR RelatingBuildingElement; HasProjections : SET [0:?] OF IfcRelProjectsElement FOR RelatingElement; ReferencedInStructures : SET [0:?] OF IfcRelReferencedInSpatialStructure FOR RelatedElements; HasPorts : SET [0:?] OF IfcRelConnectsPortToElement FOR RelatedElement; HasOpenings : SET [0:?] OF IfcRelVoidsElement FOR RelatingBuildingElement; IsConnectionRealization : SET [0:?] OF IfcRelConnectsWithRealizingElements FOR RealizingElements; ProvidesBoundaries : SET [0:?] OF IfcRelSpaceBoundary FOR RelatedBuildingElement; ConnectedFrom : SET [0:?] OF IfcRelConnectsElements FOR RelatedElement; ContainedInStructure : SET [0:1] OF IfcRelContainedInSpatialStructure FOR RelatedElements; END_ENTITY; ``` Again, notice how our **`IfcElement`** is a subtype of **`IfcProduct`** (see line 13 above). Let's continue... ``` ENTITY IfcProduct ABSTRACT SUPERTYPE OF (ONEOF (IfcAnnotation ,IfcElement ,IfcGrid ,IfcPort ,IfcProxy ,IfcSpatialStructureElement ,IfcStructuralActivity ,IfcStructuralItem)) SUBTYPE OF (IfcObject); ObjectPlacement : OPTIONAL IfcObjectPlacement; Representation : OPTIONAL IfcProductRepresentation; INVERSE ReferencedBy : SET [0:?] OF IfcRelAssignsToProduct FOR RelatingProduct; WHERE WR1 : (EXISTS(Representation) AND EXISTS(ObjectPlacement)) OR (EXISTS(Representation) AND (NOT('IFC2X3.IFCPRODUCTDEFINITIONSHAPE' IN TYPEOF(Representation)))) OR (NOT(EXISTS(Representation))); END_ENTITY; ``` Our **`IfcProduct`** is a subtype of **`IfcObject`** (see line 11 above). ``` ENTITY IfcObject ABSTRACT SUPERTYPE OF (ONEOF (IfcActor ,IfcControl ,IfcGroup ,IfcProcess ,IfcProduct ,IfcProject ,IfcResource)) SUBTYPE OF (IfcObjectDefinition); ObjectType : OPTIONAL IfcLabel; INVERSE IsDefinedBy : SET [0:?] OF IfcRelDefines FOR RelatedObjects; WHERE WR1 : SIZEOF(QUERY(temp <* IsDefinedBy | 'IFC2X3.IFCRELDEFINESBYTYPE' IN TYPEOF(temp))) <= 1; END_ENTITY; ``` Our **`IfcObject`** is a subtype of **`IfcObjectDefinition`** (see line 10 above). Let's continue... ``` ENTITY IfcObjectDefinition ABSTRACT SUPERTYPE OF (ONEOF (IfcObject ,IfcTypeObject)) SUBTYPE OF (IfcRoot); INVERSE HasAssignments : SET [0:?] OF IfcRelAssigns FOR RelatedObjects; IsDecomposedBy : SET [0:?] OF IfcRelDecomposes FOR RelatingObject; Decomposes : SET [0:1] OF IfcRelDecomposes FOR RelatedObjects; HasAssociations : SET [0:?] OF IfcRelAssociates FOR RelatedObjects; END_ENTITY; ``` Our **`IfcObjectDefinition`** is a subtype of **`IfcRoot`** (see line 5 above). We are almost done... ``` ENTITY IfcRoot ABSTRACT SUPERTYPE OF (ONEOF (IfcObjectDefinition ,IfcPropertyDefinition ,IfcRelationship)); GlobalId : IfcGloballyUniqueId; OwnerHistory : IfcOwnerHistory; Name : OPTIONAL IfcLabel; Description : OPTIONAL IfcText; UNIQUE UR1 : GlobalId; END_ENTITY; ``` Finally, we have reached the end! ### Inheritance We call this behaivor **inheritance**. * **IfcWindow** * IfcBuildingElement * IfcProduct * IfcObject * IfcObjectDefinition * IfcRoot Let's flip this "graph". This will make things a bit easier to explain. * IfcRoot * IfcObjectDefinition * IfcObject * IfcProduct * IfcBuildingElement * **IfcWindow** We can see that **`IfcRoot`** has four different attributes; *GlobalId, OwnerHistory, Name, Description.* The **`IfcObjectDefinition`** will **inherit** these attributes. This means that the **`IfcWindow`** will **inherit** these attributes. The `IfcWindow` will **inherit** all attributes from its parent entities. ## Summary If we use **inheritance**, we can see what each attribute in the **STEP-file** actually mean: ```javascript #13067= IFCWINDOW('2cXV28XOjE6f6irgi0CO$D',#42,'M_Fixed:0915 x 1830mm:353953',$,'M_Fixed:0915 x 1830mm',#35337,#13061,'353953',1830.,914.999999999999); ``` | Inhertited from | Attribute | Value | | --------------- | -------------- | ----------------------------- | | IfcRoot | GlobalId | 2cXV28XOjE6f6irgi0CO$D | | IfcRoot | Own | #42 | | IfcRoot | Name | M\_Fixed:0915 x 1830mm:353953 | | IfcRoot | Description | $ | | IfcObject | ObectType | M\_Fixed:0915 x 1830mm:353953 | | IfcProduct | ObectPlacement | #35337 | | IfcProduct | Representation | #13061 | | IfcElement | Tag | 353953 | | IfcWindow | OverallHeight | 1830. | | IfcWindow | OverallWidth | 914.999999999999999 | Or, we can simply look at the inheritance graph in the [documentation](https://standards.buildingsmart.org/IFC/RELEASE/IFC2x3/FINAL/HTML/ifcsharedbldgelements/lexical/ifcwindow.htm): ![](https://1338860669-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MSCCfuWwSmlrlrlRZ9a%2F-MSCM34TpyUsjYpuPoQu%2F-MSCMI286ucabGST8CFQ%2Fimage.png?alt=media\&token=cf446ab9-32d7-487c-aaed-bc0f083c46df)