How does select type extension behave?

[Pascal Huau]

Dear all,


a sudden doubt invades me: what is the consequence of a select type extension in a multiple schemas context?

Let's take a slightly modified example of the DAM of P11.

SCHEMA schema_1;

TYPE attachment_method = EXTENSIBLE SELECT(nail, screw);

END_TYPE;

ENTITY nail;

END_ENTITY;

ENTITY screw;

END_ENTITY;

ENTITY wall_mounting_1;

mounting : product;

on : wall;

using_method : attachment_method;

END_ENTITY;

END_SCHEMA;



SCHEMA Schema_2;

USE FROM Schema_1(wall_mounting_1);

TYPE permanent_attachment = SELECT BASED ON attachment_method WITH (glue, weld);

END_TYPE;

ENTITY glue;

END_ENTITY;

ENTITY weld;

END_ENTITY;

ENTITY wall_mounting_2;

mounting : product;

on : wall;

using_method : permanent_attachment_method;

END_ENTITY;

END_SCHEMA;



Initial remark: according to the DAM, if there was only one schema,  the attribute using_method of a valid instance of wall_mounting_1 could refer to an instance of glue

Question:

- in the 2-schemas case,  can the attribute using_method of a valid instance of wall_mounting_1 refer to an instance of glue?


My present interpretation is the following:
- there are in fact two kinds of wall_mounting_1 instances: those that are created from the definition of wall_mounting_1 in schema_1 and those that are created from the copy of wall_mounting_1 in schema_2
- for the first category of instances, the answer to my question is NO. For the 2nd, the answer is Yes.


Will this interpretation be correct with the FDAM of Express2?


NB: I suggest to provide such a 2-schemas example of select type extension in the FDAM as it corresponds to common cases of select type extension in the modules.


Last (for fun):
- Is this interpretation valid if we replace USE FROM by REFERENCE FROM?


Regards,
Pascal Huau
Association GOSET
107,111 avenue Clemenceau
92000 Nanterre
France
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