Object Relational Mapping (ORM) in computer
science is a programming technique for converting data between
incompatible type systems in object-oriented programming languages.
In object-oriented programming, data-management tasks act on object-oriented (OO) objects that are almost always non-scalar 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" with attributes/fields 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.
However,
many popular database products such as SQL database management systems (DBMS) can
only store and manipulate scalar values such as integers and strings
organized within tables. The
programmer must either convert the object values into groups of simpler values
for storage in the database (and convert them back upon retrieval), or only use
simple scalar values within the program. Object-relational mapping implements
the first approach.
The
heart of the problem involves translating the logical representation of the
objects into an atomized form that is capable of being stored in the database,
while preserving the properties of the objects and their relationships so that
they can be reloaded as objects when needed. If this storage and retrieval
functionality is implemented, the objects are said to be persistent.
Persistence
ORM
is concerned with helping your application to achieve persistence.
Persistence
means that we would like our application’s data to outlive the applications
process. In Java terms, we would like the state of (some of) our objects to
live beyond the scope of the JVM so that the same state is available later.
Relational
Databases
Specifically,
ORM is concerned with data persistence as it applies to relational databases (RDBMS). RDBMS is a very popular
persistence mechanism.
The
Object-Relational Impedance Mismatch
'Object-Relational
Impedance Mismatch' (sometimes called the 'paradigm mismatch') is a way of
saying that object models and relational models do not work very well together.
RDBMSs represent data in a tabular format, whereas object-oriented languages,
such as Java, represent it as objects. Loading and storing objects using a
tabular relational database exposes us to 5 mismatch problems…
Granularity
Sometimes
you will have an object model which has more classes than the number of corresponding
tables in the database, we says the object model is more granular than the
relational model.
Subtypes (Inheritance)
Inheritance
is a natural paradigm in object-oriented programming languages. However, RDBMSs
do not define anything similar on the whole.
Identity
A
RDBMS defines exactly one notion of 'sameness': the primary key. Java, however,
defines both object identity a==b and object equality a.equals(b).
Associations
Associations
are represented as unidirectional references in Object Oriented languages
whereas RDBMSs use the notion of foreign keys. If you need bidirectional
relationships in Java, you must define the association twice.
Likewise,
you cannot determine the multiplicity of a relationship by looking at the
object domain model.
Data navigation
The
way you access data in Java is fundamentally different than the way you do it
in a relational database. In Java, you navigate from one association to another
walking the object network.
This
is not an efficient way of retrieving data from a relational database. You
typically want to minimize the number of SQL queries and thus load several
entities via JOINs and select the targeted entities before you start walking
the object network.
Object-relational mapping
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