# Last Message Retrieval

The MessageStore::Postgres::Get::Stream::Last class is a utility that retrieves only the last message in a stream.

It's often used to retrieve the last message in a stream in order to get its position number without the use of an entity store or an entity projection.

# Example

deposit_1 = Deposited.new()
deposit_1.deposit_id = '456'

deposit_2 = Deposited.new()
deposit_2.deposit_id = '789'

stream_name = 'account-123'

Messaging::Postgres::Write.(deposit_1, stream_name)
Messaging::Postgres::Write.(deposit_2, stream_name)

last_message = MessageStore::Postgres::Get::Stream::Last.(stream_name)

last_message.deposit_id
# => "789"

last_message.metadata.position
# => 1

# Get::Stream::Last Facts

The Get::Stream::Last class returns a single message data instance representing the last message in the specified stream
The Get::Stream::Last retrieves only from entity streams, and does not work on category streams
The Get::Stream::Last can filter by a message type if the optional type parameter is specified
A Get::Stream::Last can be configured with an existing session, or it can create a new session

# MessageStore::Postgres::Get::Stream::Last Class

The Get::Stream::Last class is a concrete class from the MessageStore::Postgres library and namespace.

The Get::Stream::Last class provides:

The principle instance actuator .() (or the call instance method) for starting a reader
The class actuator .() (or the class call method) that provides a convenient invocation shortcut that does not require instantiating the reader class first

# Getting the Last Message in a Stream

A Get::Stream::Last can be actuated either via its class interface, as a matter of convenience, or via its instance interface, which allows for greater control of the configuration of the instance.

Get::Stream::Last is implemented as a callable object. Actuating it is simply a matter of invoking its call method.

# Class Actuator

self.call(stream_name, type=nil, session: nil)

Returns

Instance of MessageStore::MessageData::Read representing the last message in the stream, or nil if the stream doesn't exist.

Parameters

Name	Description	Type
stream_name	Name of stream to retrieve the last message from	String
type (optional)	Message type to filter by	String
session	An existing session object to use, rather than allowing the reader to create a new session	MessageStore::Postgres::Session

Note: The type argument is supported in Message DB from v1.3.0.

# Instance Actuator

call(stream_name, type=nil)

Parameters

Name	Description	Type
stream_name	Name of stream that the reader will read	String
type (optional)	Message type to filter by	String

Note: The type argument is supported in Message DB from v1.3.0.

# Constructing a Get::Stream::Last

The Get::Stream::Last class can be constructed in one of two ways:

Via the constructor
Via the initializer

# Via the Constructor

The constructor not only instantiates the Get::Stream::Last, but also invokes the its configure instance method, which constructs its operational dependencies.

self.build(session: nil)

Returns

Instance of the MessageStore::Postgres::Get::Stream::Last class.

Parameters

Name	Description	Type
session	An existing session object to use, rather than allowing the reader to create a new session	MessageStore::Postgres::Session

# Via the Initializer

self.initialize()

Returns

Instance of the MessageStore::Postgres::Get::Stream::Last class.

# Assigning Get::Stream::Last as a Dependency

self.configure(receiver, session: nil, attr_name: :get_last)

Constructs an instance of the Get::Stream::Last and assigns it to the receiver's get_last attribute. By default, the receiving attribute's name is expected to be get_last, but it can be altered with the use of the attr_name parameter.

something = Something.new
Messaging::Postgres::Get::Stream::Last.configure(something)

something.get_last
# => #<Messaging::Postgres::Get::Stream::Last:0x...>

Parameters

Name	Description	Type
receiver	The object that will receive the constructed `Get::Stream::Last`	Object
session	An existing session object to use, rather than allowing the reader to create a new session	MessageStore::Postgres::Session
attr_name	The receiver's attribute that will be assigned the constructed `Get::Stream::Last`	Symbol

TIP

See the useful objects user guide for background on configuring dependencies.

# Log Tags

The following tags are applied to log messages recorded by a Get::Stream::Last:

Tag	Description
get	Applied to all log messages recorded by a `Get::Stream::Last`
message_store	Applied to all log messages recorded inside the `MessageStore` namespace

The following tags may be applied to log messages recorded by a Get::Stream::Last:

Tag	Description
message_data	Applied to log messages that record the data content of a MessageData instance
data	Applied to log messages that record the data content of a MessageData instance

See the logging user guide for more on log tags.

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