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CHAPTER 5 M THE MODELING LANGUAGE
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Figure 5-5. The Car example with Engine type moved to its own module without import/export As you can see, there is an error indication (red squiggles) under the Engine type ascription and an annotation that the Engine reference can t be resolved. This shows that the Engine type definition is not visible and is outside the scope of the CarTypeExample module. You can fix this problem by adding an import directive for EngineModule within the CarTypeExample module, and an export directive for the Engine type in EngineModule. The resulting code is shown in Figure 5-6.
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Figure 5-6. The Car example with Engine type moved to its own module with import/export
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CHAPTER 5 M THE MODELING LANGUAGE
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The Engine type is now resolved within the CarTypeExample module. The import and export directives provide the means for managing modules more logically and keeping them trim. Note that the import directive refers to one or more module names. Several modules can be imported under a single directive, with the module names separated by commas, or they can be imported by separate import directives, one line for each. This is a matter of style. export directives must refer to type definitions, extents, or computed values defined within the module where the export directive is invoked. Labeled entity instances can be referenced across modules contained in separate M files and compiled in separate compilation episodes, as long as the proper export/import directives are defined. For instance, the Engine type definition shown in Figure 5-6 could have been defined in a separate M file and compiled at a different time (again, with the necessary import/export directives in the respective M files). Figure 5-10 in the following section shows the extent definitions for Cars and Engines added in the respective modules.
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Extents in M specify storage locations. In the context of SQL Server, extents correspond to SQL tables. Types in M will map to table definitions in T-SQL, but do not result in actually creating the tables in SQL Server. Code that results in a T-SQL table creation requires an extent definition. Listing 5-5 shows an example of how an extent is defined in M. Listing 5-5. Defining an Extent Cars : {Car*};
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So an extent is simply defined as a collection of zero or more type instances. If a type is to provide the basis of a SQL table definition, there must be a unique identity for each instance of the type that maps to a primary key in the SQL world. This is why you used the AutoNumber() function to define the Id of each instance of Car and Engine in the Car example (see Figure 5-7). Extents, as a matter of convention, are normally given the plural name of their contained type. Cars would be the usual name of the extent for the Car type, and Persons or People would be the usual name for a Person type, if you defined such a type. So let s expand the Car example to provide the extents needed for creating the actual Cars and Engines tables of the domain values in SQL. I ll add the extent definitions for Cars and Engines shown in Figure 5-7 (grayed lines).
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CHAPTER 5 M THE MODELING LANGUAGE
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Figure 5-7. The Car example with Cars and Engines extents ( tables) added. SQL It isn t correct, however, because the error annotation on the Engine value of the Car type definition indicates that there must be a membership constraint. This means, since there is an Engines extent defined in EngineModule, the Engine value of Car type (line 10) must be constrained to be in the Engines extent. The EngineModule code also needs to be changed to export the Engines extent, along with the Engine type (line 17), since there is now a reference to the Engines extent in the CarTypeExample module. Finally, it s generally a good idea to export types and extents defined in a module if there is a prospect that these should be exposed to other modules as you continue to refine your model. So you still add a declaration to export the Car type and Cars extent, as shown in line 4 of Figure 5-8 (which shows the corrected code).
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