Add Fields documentation

This commit includes markdown and sample files related to fields support in Protean.

docs/guides/compose-a-domain/activate-domain.md

@@ -1,5 +1,7 @@
 # Activate the domain
 
+Once a domain object is defined, the next step is to activate it.
+
 A `Domain` in protean is always associated with a domain context, which can be
 used to bind an domain object implicitly to the current thread or greenlet. We
 refer to the act of binding the domain object as **activating the domain**.

docs/guides/domain-definition/fields/association-fields.md

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+# Association Fields
+
+## `HasOne`
+
+Represents an one-to-one association between an aggregate and its entities.
+This field is used to define a relationship where an aggregate is associated
+with at most one instance of a child entity.
+
+```python hl_lines="10 13"
+{! docs_src/guides/domain-definition/fields/association-fields/001.py !}
+```
+
+!!!note
+    If you carefully observe the `HasOne` field declaration, the child entity's
+    name is a string value! This is usually the way to avoid circular references.
+    It applies to all aspects of Protean that link two entities - the string
+    value will be resolved to the class at runtime.
+
+The `Author` entity can now be persisted along with the `Book` aggregate:
+
+```shell hl_lines="3 12-13"
+In [1]: book = Book(
+   ...:     title="The Great Gatsby",
+   ...:     author=Author(name="F. Scott Fitzgerald")
+   ...: )
+
+In [2]: domain.repository_for(Book).add(book)
+Out[2]: <Book: Book object (id: a4a642d9-87ed-44de-9889-c687466f171b)>
+
+In [3]: domain.repository_for(Book)._dao.query.all().items[0].to_dict()
+Out[3]: 
+{'title': 'The Great Gatsby',
+ 'author': {'name': 'F. Scott Fitzgerald',
+  'id': '1f275e92-9872-4d96-b999-4ef0fbe61013'},
+ 'id': 'a4a642d9-87ed-44de-9889-c687466f171b'}
+```
+
+!!!note
+    Protean adds a `Reference` field to child entities to preserve the inverse
+    relationship - from child entity to aggregate - when persisted. This is
+    visible if you introspect the fields of the Child Entity.
+
+    ```shell hl_lines="7 13"
+    In [1]: from protean.reflection import declared_fields, attributes
+
+    In [2]: declared_fields(Author)
+    Out[2]: 
+    {'name': String(required=True, max_length=50),
+    'id': Auto(identifier=True),
+    'book': Reference()}
+
+    In [3]: attributes(Author)
+    Out[3]: 
+    {'name': String(required=True, max_length=50),
+    'id': Auto(identifier=True),
+    'book_id': _ReferenceField()}
+    ```
+
+We will further review persistence related aspects around associations in the
+Repository section.
+<!-- FIXME Link Repository section -->
+
+## `HasMany`
+
+Represents a one-to-many association between two entities. This field is used
+to define a relationship where an aggregate has multiple instances of a child
+entity.
+
+```python hl_lines="11"
+{! docs_src/guides/domain-definition/fields/association-fields/002.py !}
+```
+
+Protean provides helper methods that begin with `add_` and `remove_` to add
+and remove child entities from the `HasMany` relationship.
+
+```shell hl_lines="4-5 12-13 16 23"
+In [1]: post = Post(
+   ...:     title="Foo",
+   ...:     comments=[
+   ...:         Comment(content="Bar"),
+   ...:         Comment(content="Baz")
+   ...:     ]
+   ...: )
+
+In [2]: post.to_dict()
+Out[2]: 
+{'title': 'Foo',
+ 'comments': [{'content': 'Bar', 'id': '085ed011-15b3-48e3-9363-99a53bc9362a'},
+  {'content': 'Baz', 'id': '4790cf87-c234-42b6-bb03-1e0599bd6c0f'}],
+ 'id': '29943ac9-a9eb-497b-b6d2-466b30ecd5f5'}
+
+In [3]: post.add_comments(Comment(content="Qux"))
+
+In [4]: post.to_dict()
+Out[4]: 
+{'title': 'Foo',
+ 'comments': [{'content': 'Bar', 'id': '085ed011-15b3-48e3-9363-99a53bc9362a'},
+  {'content': 'Baz', 'id': '4790cf87-c234-42b6-bb03-1e0599bd6c0f'},
+  {'content': 'Qux', 'id': 'b1a7aeda-81ca-4d0b-9d7e-6fe0c000b8af'}],
+ 'id': '29943ac9-a9eb-497b-b6d2-466b30ecd5f5'}
+```

docs/guides/domain-definition/fields/container-fields.md

@@ -0,0 +1,105 @@
+# Container Fields
+
+## `List`
+
+A field that represents a list of values.
+
+**Optional Arguments**
+
+- **`content_type`**: The type of items in the list. Defaults to `String`.
+Accepted field types are `Boolean`, `Date`, `DateTime`, `Float`, `Identifier`,
+`Integer`, `String`, and `Text`.
+- **`pickled`**: Whether the list should be pickled when stored. Defaults to
+`False`.
+
+!!!note
+    Some database implementations (like Postgresql) can store lists by default.
+    You can force it to store the pickled value as a Python object by
+    specifying `pickled=True`. Databases that don’t support lists simply store
+    the field as a python object.
+
+```python hl_lines="9"
+{! docs_src/guides/domain-definition/fields/simple-fields/001.py !}
+```
+
+The value is provided as a `list`, and the values in the `list` are validated
+to be of the right type.
+
+```shell hl_lines="6 12"
+In [1]: user = User(email="[email protected]", roles=['ADMIN', 'EDITOR'])
+
+In [2]: user.to_dict()
+Out[2]: 
+{'email': '[email protected]',
+ 'roles': ['ADMIN', 'EDITOR'],
+ 'id': '582d946b-409b-4b15-b3be-6a90284264b3'}
+
+In [3]: user2 = User(email="[email protected]", roles=[1, 2])
+ERROR: Error during initialization: {'roles': ['Invalid value [1, 2]']}
+...
+ValidationError: {'roles': ['Invalid value [1, 2]']}
+```
+
+## `Dict`
+
+A field that represents a dictionary.
+
+**Optional Arguments**
+
+- **`pickled`**: Whether the dict should be pickled when stored. Defaults to
+`False`.
+
+```python hl_lines="10"
+{! docs_src/guides/domain-definition/fields/container-fields/002.py !}
+```
+
+A regular dictionary can be supplied as value to payload:
+
+
+```shell hl_lines="3 9"
+In [1]: event=UserEvent(
+   ...:     name="UserRegistered",
+   ...:     payload={'name': 'John Doe', 'email': '[email protected]'}
+   ...: )
+
+In [2]: event.to_dict()
+Out[2]: 
+{'name': 'UserRegistered',
+ 'payload': {'name': 'John Doe', 'email': '[email protected]'},
+ 'id': '44e9143f-f4a6-40da-9128-4b6c013420d4'}
+```
+
+!!!note
+    Some database implementations (like Postgresql) can store dicts as JSON
+    by default. You can force it to store the pickled value as a Python object
+    by specifying pickled=True. Databases that don’t support lists simply store
+    the field as a python object.
+
+## `ValueObject`
+
+Represents a field that holds a value object. This field is used to embed a
+Value Object within an entity.
+
+**Arguments**
+
+- **`value_object_cls`**: The class of the value object to be embedded.
+
+```python hl_lines="7-15 20"
+{! docs_src/guides/domain-definition/fields/container-fields/003.py !}
+```
+
+You can provide an instance of the Value Object as input to the value object
+field:
+
+```shell hl_lines="2 8"
+In [1]: account = Account(
+   ...:     balance=Balance(currency="USD", amount=100.0),
+   ...:     name="Checking"
+   ...: )
+
+In [2]: account.to_dict()
+Out[2]: 
+{'balance': {'currency': 'USD', 'amount': 100.0},
+ 'name': 'Checking',
+ 'id': '513b8a78-e00f-45ce-bb6f-11ef0cccbec6'}
+```