A Schevo database is the combination of a schema definition, stored data, and an engine that exposes the two. This document describes in detail the schema definition syntax used by Schevo.
Schema definitions are pure-Python modules that make use of Schevo’s syntax extensions. Schema definitions can be directly imported anywhere, but typically the import process is managed by the Schevo database engine in order to tie a schema to an open database.
The database engine is defined in the schevo.database module, and provides read/write access to stored data using the higher-level structures defined by the database’s schema definition.
This document also functions as a doctest:
>>> from schevo.test import DocTest
This section describes the syntax used for schema definitions.
The following cannot be used for field names in Entity class definitions:
All schemata must begin with the following two lines:
from schevo.schema import * schevo.schema.prep(locals())
A file is provided in the Schevo source distribution called SCHEMA-PREAMBLE.txt that contains the source code required for all Schevo schemata, as well as some additional comments that assist in starting a new database schema.
To include icon support in a database schema, add the following extent definition:
class SchevoIcon(E.Entity): _hidden = True name = f.string() data = f.image() _key(name)
A Schevo database schema consists of a series of the following types of declarations:
Code blocks defined in a schema make use of the Database API.
The following objects are available in the global Python namespace of a schema module:
The following Schevo-managed namespaces are available in the global Python namespace of a schema module.
Methods defined in the schema definition may use any part of the public API exposed by the currently opened database that is associated with the schema.
These namespaces cannot be directly modified. Schevo manages them, and automatically adds objects to them when it imports a schema definition as a module.
An entity/extent definition minimally consists of a subclass of E.Entity:
class Foo(E.Entity): """Description of Foo."""
The above example defines a Foo extent, which contains Foo entities that each have zero fields, as none were defined.
The entity/extent definition may further contain the following types of declarations:
Field definitions. Here is a Foo extent where each entity has a name field of string type, and a FooChild extent where each entity has a reference to a Foo entity and also has a bar field of integer type:
class Foo(E.Entity): name = f.string() class FooChild(E.Entity): foo = f.entity('Foo') bar = f.integer()
Calculated field methods. Here is a Gender extent whose entities have a count field of type integer that calculates the number of Person entities whose gender field references that Gender entity:
class Gender(E.Entity): code = f.string() name = f.string() @f.integer() def count(self): return self.s.count('Person', 'gender') class Person(E.Entity): name = f.string() gender = f.entity('Gender', required=False)
Key/index specifications. Here is a Person extent whose entities must have unique values for their name field, and that is also indexed by age then name:
class Person(E.Entity): name = f.string() age = f.integer() _key(name) _index(age, name)
Entity/extent-specific extension methods.
Extent labels. Schevo creates a default singular and plural label for extents based on the class name used to define the extent. This may be overridden in the class definition. Here is a Person extent whose plural label is “People”:
class Person(E.Entity): _plural = u'People'
Here is a TpsReport extent whose singular label is “T.P.S. Report”. The default plural label is based on the singular label, so the plural label for this extent is “T.P.S. Reports”:
class TpsReport(E.Entity): _label = u'T.P.S. Report'
Entity string representations. Schevo user interfaces make use of string representations of entities when presenting short summaries of them. The default string representation of an entity is the value of its name field if it has one, or the result of calling repr() on the entity if not. Here is an example of a FooChild extent that has a custom string representation:
class Foo(E.Entity): name = f.string() class FooChild(E.Entity): foo = f.entity('Foo') bar = f.integer() def __unicode__(self): return u'%s :: %s' % (self.foo, self.bar)
In the above example, if a FooChild entity’s bar field value was 12, and its foo value referenced a Foo entity whose name was u'abc', the result of calling unicode() on the FooChild entity would be u'abc :: 12'.
Extent initial/sample data.
Initial data is data that Schevo uses when creating a new database. Initial data is always processed during new database creation. Specify initial data for an extent by assigning an _initial attribute to the entity class in your database schema.
Sample data is data that Schevo uses to populate a database after its creation. Sample data is only processed if the -p or --sample option is passed to the schevo db create command-line tool, or if you call the populate method on an open database. Specify sample data for an extent by assigning a _sample attribute to the entity class in your database schema.
Unit test sample data is data that Schevo populates a database with when running a suite of unit tests against a database schema. Specify unit test sample data for an extent by assigning a _sample_unittest attribute to the entity class in your database schema.
You may also specify sample data with a custom attribute name, such as _sample_abc. Populate a database with your custom sample data by passing the suffix to the call to populate on your database. For the example, to populate db with the abc sample data, call db.populate('abc').
Specify each collection of initial or sample data as a list of tuples, where each tuple contains values for the fields that the extent’s create transaction expects, in the order that it expects them. Normally, those fields are the same as the fields specified in the entity class, but in complex database schemata the fields may differ.
To specify the default value for a field, use the constant DEFAULT. For example, the Foo entities that have the names 'c' and 'd' will have a size of 5:
class Foo(E.Entity): name = f.string() size = f.integer(default=5) _key(name) _initial = [ ('a', 1), ('b', 2), ('c', DEFAULT), ('d', DEFAULT), ]
To specify a value for an Entity field that allows only one entity type, use a tuple containing the values of the fields of the first key of that entity type. For example:
class Foo(E.Entity): name = f.string() for_rainy_days = f.boolean() _key(name, for_rainy_days) _initial = [ ('One', False), ('Two', False), ('Buckle', False), ('Shoe', False), ('Shoe', True), ] class Bar(E.Entity): foo = f.entity('Foo') baz = f.string() _key(foo, bar) _initial = [ (('One', False), 'This is how it starts.'), (('Shoe', False), 'This is how it ends.'), (('Shoe', False), 'Need to have two shoes.'), (('Shoe', True), 'And one for a rainy day.'), ]
To specify a value for an Entity field that allows more than one entity type, use a two-tuple that first gives the entity type, then gives a tuple of the values of the fields of the first key of that entity type. For example:
class Foo(E.Entity): name = f.string() _key(name) _initial = [ ('one',), ('two',), ] class Bar(E.Entity): number = f.integer() _key(number) _initial = [ (1,), (2,), ] class Baz(E.Entity): foo_or_bar = f.entity('Foo', 'Bar') notes = f.string() _initial = [ (('Foo', ('one',)), 'This is the Foo that is named one.'), (('Foo', ('two',)), 'This is the Foo that is named two.'), (('Bar', (1,)), 'This is the Bar that is numbered 1.'), (('Bar', (2,)), 'This is the Bar that is numbered 2.'), ]
To specify values for EntityList and EntitySet fields, simply enclose the representations of entities within a list or a set, respectively.
Define a field for an Entity, Parameterized Query, Transaction, or View by using a field factory. Field factories are accessed using the global f namespace.
The members of the f namespace are all of the available field types, converted from their “CamelCase” names to “lowercase_with_underscores” names, e.g. the Boolean field class becomes f.boolean, and the EntityList field class becomes f.entity_list.
For example, this Entity subclass defines three fields:
class Dog(E.Entity): name = f.string() #  birthdate = f.date(required=False) #  disposition = f.entity('Disposition', on_delete=UNASSIGN, required=False, default=('Cheerful',)) # 
The name field is a required String field. By default, all fields are required.
The birthdate field is an optional Date field.
The disposition field is an optional Entity field that can store a reference to a Disposition entity.
When the referenced Disposition entity is deleted, this field’s value is set to UNASSIGNED. See also cascading delete rules for Entity fields.
The default value of this field is the Disposition entity that matches ('Cheerful',). See Default field values below.
When defining an Entity, EntityList, EntitySet, or a custom entity-storing field, you may specify cascade delete rules for that field.
Upon receiving a request to delete an entity whose reference is stored in one of these fields, Schevo will first check the rules of those fields to see if the deletion is allowed and, if so, what should happen to the field or entity that refers to the deleted entity.
The default rule of the available rules is RESTRICT, which is the safest operation, since it prevents accidental deletion of an entity if it is being referred to by another entity.
Default field values are assigned to fields in Create transactions.
Specify default field values in a field definition by giving either a default value in the same notation as you would for initial or sample data, or by specifying a callable that returns the actual value to be used as the default value.
Default values are not assigned to a field if a value has been supplied for that field in the keyword arguments specified when creating the Create transaction.
Specify a rule for only a certain type of entity by giving the type name and rule as a tuple to the field factory.
For example, to specify an Entity field that can refer to a Cog or a Sprocket that allows cascade deletion when a Cog is deleted but disallows deletion of a referred-to Sprocket, use this:
part = f.entity(('Cog', CASCADE), ('Sprocket', RESTRICT))
Specify a rule for all types of entities by giving a value to the on_delete keyword argument to the field factory.
For example, to specify an Entity field that can refer to a Cog, a Sprocket, or a Widget, but that requests field unassignment on deletion of the referred entity, use this:
part = f.entity('Cog', 'Sprocket', 'Widget', on_delete=UNASSIGN)
CASCADE: If entity B has a field that refers to entity A, and entity A is deleted, entity B will be deleted as well, provided that no other rules are restricting the deletion of entity B.
REMOVE: If entity B has an EntityList or EntitySet field that contains a reference to entity A, and entity A is deleted, the reference to A will be removed from that field in B.
RESTRICT: If entity B has a field that refers to entity A, Schevo will not allow deletion of entity A.
UNASSIGN: If entity B has a field that refers to entity A, and entity A is deleted, the field containing the reference to A will be set to UNASSIGNED, if allowed.
If the field definition in B is an Entity field, it must include required=False. If the field in B is a required field, then the unassignment will not work and the deletion will be restricted.
If the field definition in B is an EntityList field, it must include allow_unassigned=True. If the field in B does not allow UNASSIGNED to be a member of its collection, then the unassignment will not work and the deletion will be restricted.
There may be complex interactions between cascade deletion rules if you have complex logic in your application. Schevo is designed to handle these in a consistent manner. As of this writing, please see the source for schevo.transaction, and the source for the test_on_delete unit test, for further information.
Schevo contains many built-in field types typically used when building a database schema. You can also create custom field types when your schema has unique requirements for data types.
String fields are used to store Unicode strings. An application may assume that a String field’s value can be displayed to a user as text.
String fields accept a multiline attribute that you can set to one of the following:
Path fields are String fields used to store filesystem paths.
Bytes fields are used to store 8-bit byte sequences. An application should not assume that a Bytes field can be displayed to a user as text.
Image fields are Bytes fields that store binary data for an image.
Integer fields store integer values.
Float fields store floating point values.
Money fields store fixed-point fractional values, and are commonly used to store values representing monetary amounts.
Date fields store datetime.date objects.
Datetime fields store datetime.datetime objects.
A Boolean field stores a boolean value.
An Entity field stores a reference to another entity.
An EntityList field stores a list of references to other entities.
An EntitySet field stores a set of references to other entities.
A HashedValue field, upon having its value set, will store a one-way hash of that value.
Use the compare(value) method of a HashedValue field instance to see if the hash matches a value.
It is not possible to retrieve the original value from the one-way hash.
HashedPassword fields are a convenience class to show that a field specifically stores hashed values of passwords.
This is useful for storing information about a user’s password in a reasonably secure manner, preventing immediate discovery of passwords if a proprietary Schevo database were stolen or otherwise accessed by unauthorized users.
Deprecated field types are those field types that are available in Schevo in order to support legacy schemata, but are no longer recommended for use.
If you have a database schema that uses any of these field types, you should use a different field type instead.
When you use a deprecated field type, Schevo will give a Python DeprecationWarning about such use to encourage you to use a different field type.
The following types of fields are deprecated:
Blob: Use Bytes field types instead.
Memo: Use String field types instead, setting multiline=True in your field definition.
Password: Use one of the HashedValue field types instead.
If you wish to store a plain-text password in your schema, you can do one of the following:
Use the String field type:
class Foo(E.Entity): password = f.string()
Create a custom PlaintextPassword field type:
class PlaintextPassword(F.String): pass class Foo(E.Entity): password = f.plaintext_password()
Add query methods to entity class definitions to provide access to query classes.
For entity-level queries, a query method looks something like this, where factor is an optional argument that can be given when the query method is called, and RelatedStuff is a global query class defined elsewhere:
def q_related_stuff(self, factor=None): q = RelatedStuff() if factor is not None: q.factor = factor return q
Simple queries are queries that do not take any parameters and are not visibly composed of subqueries.
Because of these properties, it is easy to embed a function within a query method to expose simple queries, without having to write a separate query class to hold the code that performs the query.
An embedded simple query looks something like this:
def q_related_stuff(self): def fn(): # ... Do stuff to build "results" ... return results return Q.Simple(fn, 'Related Stuff')