In the EC object model a relationship class is a descendant of an ordinary class. It therefore has properties and can contain data. Hence an ECRelationship mapping is also a descendant of an ECClass mapping. Necessarily all operations previously described for the ECClass maping are possible for the relationship mapping too.

Types of Relationships

Natively a database maintains relationships via Foreign Keys. If the cardinality of relationship is defined 1 to many the Foreign Key is often a quite convenient way of storing such a relationships in a database.

Another case of cardinality is many to many. These are mostly stored in a separate table or tables and sometimes have their own properties.

Both types of relationships are supported by the Plant Project Database Mapping Extension.

Mapping of Foreign Key relationships

Let's take a look at a mapping of a Foreign Key relationship. For example select the "EQUIPMENT_HAS_NOZZLE" relationship in the relationship class tree and click the "Edit" button or right click on the relationship class and select "Add Mapping..." from the context menu. The Edit relationship Mapping dialog box is displayed. This dialog box is very similar to the Edit Class Mapping dialog box but has one additional section "Relationship" that contains mappings for Keys of the Source and Target related classes. To create a mapping of a Foreign Key relationship it is necessary to perform the following steps:

• At first select the type of relationship in the "Storing Type" combo box. It is "FOREIGN_KEY" in our case.
• After that it is necessary to provide the table where primary key value of the related object is contained in. For "EQUIPMENT_HAS_NOZZLE" the ID of EQUIPMENT is stored in the NOZZLE table:
  Add "NOZZLE" table.

• After that it is necessary to define either the Source or the Target Key mapping.
  • If the ID of the Source relationship constraint class is in the table of the Target class then define the "Source Key"
  • If the ID of the Target relationship constraint class is in table of the Source class then define the "Target Key"

In our example the Source constraint class is "EQUIPMENT" and the Target class is "NOZZLE". The ID of EQUIPMENT is in the Target's table. Thus it is necessary to define the "Source Key". Select "NOZZLE" as table and "NEQUIP" as column

Note: Only one of the mappings either for the Source Key or for the Target Key should be provided for the Foreign Key relationship.

Note: When a Foreign Key relationship mapping is created it is also optional possible to map ID, Name and Label and other properties similar to the Class Mapping. In practice such relationships are stored in a single field and even the InstanceID mapping cannot be provided. For such cases the InstanceID is auto-generated by the following rule:

<Source Instance ID>.<Target Instance ID>.<ECClass Name>

Note: The mapping of the Source Key and the Target Key allows involving two or more columns.

Note: The value of the Source and Target Key should be the same as the Instance ID of the Source or Target instances. E.g. the value of NOZZLE.NEQUIP_GUID_PK should be equal to the InstanceID of EQUIPMENT. Otherwise reading and writing of such a relationship won't work.

Mapping relationships stored in a separate table

For relationships that are stored in a separate table everything that is possible for a class is possible as well. So all steps described in Creation of New Class Mapping should be done as well. Let's create a mapping for such a relationship e.g. for the "DOCUMENT_HAS_PIPELINE" relationship (see Fig.30):

• Open the Edit Relationship mapping dialog box
• Define Storing Type: "SEPARATE_TABLE"
• Define the tables where the relationships are stored. These are "PROC_LNK" and "KEY_LINK"
• Map the InstanceID. This relationship has its own ID so a mapping is necessary. The ID is mapped to "PROC_LNK.LINK_ID".
• Map Source and Target Keys:
  • Source Key (field with the ID of DOCUMENT): PROC_LNK.DWG_NAME
  • Target Key (field with the ID of PIPELINE): KEY_LINK.KEY_TAG

• Once the relationship is stored in the two tables it is necessary to define the join rules (see Fig.31):
  • Joined Table Name: PROC_LNK
  • Joined Table Column Name: LINK_ID
  • Main Table Name: KEY_LINK
  • Main Table Column Name: LINK_ID
    

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