I am not re-defining terms - I understand them correctly. An ORM is a "Mapping" between "Objects", and "Relations".
Here are the parts of the wikipedia pages, that are actually-relevant* * to this discussion (important parts in bold-text): *ORM:* "...Data management <http://en.wikipedia.org/wiki/Data_management> tasks in object-oriented (OO) programming are typically implemented by manipulating * objects <http://en.wikipedia.org/wiki/Object_(computer_science)> that are almost always non-scalar <http://en.wikipedia.org/wiki/Scalar_(computing)>** values.* For example, consider an address book entry that represents *a single person along with zero or more phone numbers and zero or more addresses*. This could be modeled in an object-oriented implementation by a "Person object <http://en.wikipedia.org/wiki/Object_(computer_science)>" with attributes/fields <http://en.wikipedia.org/wiki/Attribute_(computing)> to hold each data item that the entry comprises: the person's name, *a list of phone numbers, and a list of addresses.* The list of phone numbers would itself *contain "PhoneNumber objects" *and so on. The address book entry is treated as a single object by the programming language (it can be referenced by a single variable containing a pointer to the object, for instance). *Various methods can be associated with the object, such as a method to return the preferred phone number, the home address*, and so on." The first thing to notice here, is that an ORM object-attribute can contain NON-SCALAR values - meaning, a link to a list of other objects. There is no feature in web2py that generates such an object. The second thing to notice here, is that the attributes of an ORM object usually contain child-objects (plural) that represent fields from a different table than the parent-object. Again, there is no feature in web2py that can generate such an object. A JOIN operation may return row objects, each of which may contain sub-attributes that hold A SINGLE field-value from a foreign-table, but it is a *scalar-value* - NOT another * domain-entity-object* (with it's own attributes, etc.), NOR a SEQUENCE of *domain-entity objects* Here are some diagrams that presents it really well: <http://www.jfwk.com/images/p2.gif> <http://software-carpentry.org/3_0/summary/orm.png> <http://www.visual-paradigm.com/VPGallery/img/orm/ERDiagramAndClassDiagramSynchronization/ER-Diagram-and-Class-Diagram-Synchronization-Sample.png> The crucial thing to notice here is that an ORM object-attribute can contain NON-SCALAR values - meaning, a link to a list of other objects, which themselves may contain links to other objects/sequences-of-objects, etc. As for DAL, here is the part of the wilipedia page, that is relevant to this discussion: "...*Popular use for database abstraction layers* are among object-oriented programming <http://en.wikipedia.org/wiki/Object-oriented_programming> languages, which are similar to API level abstraction layers. In an object oriented language like C++ or Java, *a database can be represented through an ** object* <http://en.wikipedia.org/wiki/Object_(computer_science)>*, whose methods and members* (or the equivalent thereof in other programming languages)* represent various functionalities of the database.* They also share the same advantages and disadvantages as API level interfaces." As you can see, even wikipedia says that there is more to a DAL than just the SQL translation. Here is another usage of the same "Three Letter Acronym" (DAL), that represents how an ORM is layered on-top of a DAL: <http://vaadin.com/download/book-of-vaadin/vaadin-7/html/img/jpacontainer/jpa-mapping-graphic-lo.png> <http://vaadin.com/download/book-of-vaadin/vaadin-7/html/img/jpacontainer/three-layer-architecture-lo.png> Obviously, THIS dal is not an abstraction-layer, but an access-layer, but it could very-well be substituted by an abstraction-layer that does the same thing. Granted, this is a different form of an ORM, directly mapping class-attributes to table-fields, but in principal it is the same - an ORM is a layer on-top a DAL, that uses a DAL. -- --- You received this message because you are subscribed to the Google Groups "web2py-users" group. To unsubscribe from this group and stop receiving emails from it, send an email to web2py+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.
