OrientDB is a Graph Database "on steroids" because it supports concepts taken from both the Document Database and Object-Oriented worlds. This Tutorial step shows the power of Graphs used in conjunction with schema and constraints.
Take a look at this use case: Creating a graph to map the relationships between Person and Cars.
Let's open a shell (or command prompt in Windows) and launch the OrientDB Console (use console.bat on Windows):
> cd $ORIENTDB_HOME/bin
> ./console.sh
Now we're going to use the console to create a brand new local database:
orientdb> create database plocal:../databases/cars admin admin plocal
Ok, now let's create the first graph schema with "Person" and "Car" as 2 new Vertex types and "Owns" as an Edge type:
orientdb> create class Person extends V
orientdb> create class Car extends V
orientdb> create class Owns extends E
And let's go populate the database with the first Graph elements:
orientdb> create vertex Person set name = 'Luca'
Created vertex 'Person#11:0{name:Luca} v1' in 0,012000 sec(s).
orientdb> create vertex Car set name = 'Ferrari Modena'
Created vertex 'Car#12:0{name:Ferrari Modena} v1' in 0,001000 sec(s).
orientdb> create edge Owns from (select from Person) to (select from Car)
Created edge '[e[#11:0->#12:0][#11:0-Owns->#12:0]]' in 0,005000 sec(s).
Ok, now we can traverse vertices. For example, what is Luca's car? Traverse from Luca vertex to the outgoing vertices following the "Owns" relationships:
orientdb> select name from ( select expand( out('Owns') ) from Person where name = 'Luca' )
----+-----+--------------
# |@RID |name
----+-----+--------------
0 |#-2:1|Ferrari Modena
----+-----+--------------
Perfect. Now we want to have the location of each Person. We need another Vertex type called "Country" to connect to the Person with a new "Lives" Edge type:
orientdb> create class Country extends V
orientdb> create class Lives extends E
orientdb> create vertex Country set name = 'UK'
Created vertex 'Country#14:0{name:UK} v1' in 0,004000 sec(s).
Next, let's associate Luca with the UK Country:
orientdb> create edge Lives from (select from Person) to (select from Country)
Created edge '[e[#11:0->#14:0][#11:0-Lives->#14:0]]' in 0,006000 sec(s).
So far so good. Our graph has been extended. Now, try to search the country where there are "Ferrari" cars in our database.
orientdb> select name from ( select expand( in('Owns').out('Lives') ) from Car where name like '%Ferrari%' )
----+-----+----
# |@RID |name
----+-----+----
0 |#-2:1|UK
----+-----+----
Now we've modeled our graph using a schema without any constraints. But it would be useful to require an Owns relationship to exist only between the Person and Car vertices. So, let's create these constraints:
orientdb> create property Owns.out LINK Person
orientdb> create property Owns.in LINK Car
The MANDATORY setting against a property prevents OrientDB from using a lightweight edge (no physical document is created). Be sure to pay attention and not put spaces between MANDATORY=true.
orientdb> alter property Owns.out MANDATORY=true;
orientdb> alter property Owns.in MANDATORY=true;
If we want to prohibit a Person vertex from having 2 edges against the same Car vertex, we have to define a UNIQUE index against out and in properties.
orientdb> create index UniqueOwns on Owns(out,in) unique
Created index successfully with 0 entries in 0,023000 sec(s).
Unfortunately, the index tells us 0 entries are indexed. Why? We have already created the Owns relationships between "Luca" and "Ferrari Modena." In that case, OrientDB had already created a lightweight edge before we set the rule to force creation documents for Owns instances. So, you need to drop and recreate the edge.
orientdb> delete edge from #11:0 to #12:0
orientdb> create edge Owns from (select from Person) to (select from Car)
Now check that the record has been created.
orientdb> select from Owns
----+-----+-----+-----
# |@RID |out |in
----+-----+-----+-----
0 |#13:0|#11:0|#12:0
----+-----+-----+-----
So far so good. The constraints works. Now try to create a "Owns" edge between Luca and UK (Country vertex):
orientdb> create edge Owns from (select from Person) to (select from Country)
Error: com.orientechnologies.orient.core.exception.OCommandExecutionException: Error on execution of command: sql.create edge Owns from (select from Person) to (sel...
Error: com.orientechnologies.orient.core.exception.OValidationException: The field 'Owns.in' has been declared as LINK of type 'Car' but the value is the document #14:0 of class 'Country'
Now we have a typed graph with constraints.
For more information look at Graph Schema.