# APIMAS Predicates¶

As explained in the previous section, an APIMAS specification contains structural elements with their respective metadata. This metadata are prefixed with a dot ‘.’ and called Predicates. Predicates add semantics to their parent structural elements and therefore, it is a way to change the behaviour of your application.

APIMAS defines a set of predicates whose semantics are understood from every application (both client and server side) and help you create your specification. Below, there is a list of the widely-known predicates:

## Structural predicates¶

The predicates listed below describe basic structural elements of your REST application.

Predicate Description
.endpoint It defines a location to the web after which there is a set of collections to interact.
.collection It defines that the parent node is a collection of resources of the same type, where each resource can be related to other resources, it is described by some data, and there are actions that can be performed on it.

## Action predicates¶

There are also predicates which delineate what actions or methods can be performed on a collection or a resource individually.

Predicate Description
.list

The list of resources contained to the collection is permitted. It corresponds to:

GET <collection name>/

.retrieve

A single resource can be retrieved and viewed. It corresponds to:

GET <collection name>/<pk>/

.create

A new resource of the type defined by the collection can be created. It corresponds to:

POST <collection name>/

.update

A single resource can be updated. It corresponds to:

PUT <collection name>/<pk>/ PATCH <collection name>/<pk>/

.delete

A single resource can be deleted from the set of existing. It corresponds to:

DELETE <collection name>/<pk>/

Note

The .update predicate allows a single resource to be both replaced and partially updated.

Action predicates are specified inside the structural element actions of a collection definition.

Example:

{
'foo': {
'.collection': {},
'actions': {
'.list': {},
'.retrieve': {},
}
}
}


In the above example, only retrieve and list operations are permitted for collection ‘foo’.

## Resource description¶

Each resource contained in a particular collection is described by a field schema with properties and data associated with it. Specifically, each resource is described by a set of fields with specific type and behaviour.

Similarly to the structural and action predicate, there are also predicates to describe the properties of every field. These predicates slit into two categories: a): types, b): properties.

### Type Predicates¶

Predicate Description
.integer Parent node is an integer.
.float Parent node is a floating point number.
.string

Parent node is a string. Small to middle sized strings are supported.

Parameters:
maxlength: The upper bound of string’s size (optional). The default is 255.
.text Parent node is a text.
.boolean Parent node is either true or false.
.email Parent node is an email address.
.serial Parent node is a serial, non-writable integer number.
.choices

Parent node can take a list of allowed values as specified by the parameter allowed.

Example:

'foo': {
'.choices': {
'allowed': [1, 'bar']
}
}


Parent node can be either 1 or ‘bar’.

Parameters:

allowed: A list of acceptable values for the parent. (Each item must be a literal).

display: A list of the displayed values of the node (optional). By default is the same as allowed.

.ref

Parent node points to the web location of another resource.

Parameters:

to: Name of the collection where resource is located. This must be a valid name of a collection which have been specified on APIMAS specification too.

many: true if parent node points to multiple resources, false otherwise.

.identity Parent node points to the web location of this resource. It’s actually the REST identifier of the resource. It is non-writable.
.file Parent node is a file.
.date

Parent node is a date, represented by a string.

Parameters:
format: A list of string representing the allowed input formats of the date. (optional). By default only ISO-8601 is allowed.
.datetime

Parent node is a datetime, represented by a string.

Parameters:
format: A list of strings representing the allowed. input formats of the datetime. By default only ISO-8601 is allowed.
.struct

Parent node is a structure which consists of another field schema, i.e. a set of fields with their types and properties.

Arguments:
A document-like representation with the name of fields as key and their description as defined by the use of predicates.
.structarray

Parent node is an array of structures.

Arguments:
A document-like representation with the name of fields as key and their description as defined by the use of predicates.

Note

Every field must be described with at most one type.

### Properties predicates¶

Properties predicates, typically, describe the behaviour and how can be used on the various actions.

Predicate Description
.required The parent node is required and must be included in every API call associated with create and update operations (e.g. POST and PUT requests).
.readonly The parent node is read-only and its value can be viewed, but it cannot be modified or set.
.writeonly The parent node is write-only and its value can be modified or set, but it cannot be viewed.
.nullable The parent node can have null values.

Note

Some predicates are mutually exclusive. Specifically a node cannot be described as both .readonly and writeonly or .required and .readonly.