# Projection Fixture

The projection fixture tests the projection of event messages onto an entity. It tests that the attributes of an event are copied to the entity. The attributes tested can be limited to a subset of attributes by specifying a list of attribute names. A map can be provided to compare attributes that have a different name on the source event than on the entity. The projection fixture also allows the testing of values copied from an event that are transformed before being assigned to an entity's attributes. The copy-and-transform assertion can also accept a map to test the transformation between attributes that have a different name on the source event than on the entity.

# Example

class SomeEntity
  include Schema::DataStructure

  attribute :id, String
  attribute :amount, Numeric, default: 0
  attribute :time, ::Time
  attribute :other_time, ::Time
end

class SomeEvent
  include Messaging::Message

  attribute :example_id, String
  attribute :amount, Numeric, default: 0
  attribute :time, String
  attribute :some_time, String
end

class SomeProjection
  include EntityProjection

  entity_name :some_entity

  apply SomeEvent do |some_event|
    some_entity.id = some_event.example_id
    some_entity.amount = some_event.amount
    some_entity.time = Time.parse(some_event.time)
    some_entity.other_time = Time.parse(some_event.some_time)
  end
end

context "SomeProjection" do
  some_event = SomeEvent.new
  some_event.example_id = SecureRandom.uuid
  some_event.amount = 11
  some_event.time = Time.utc(2000)
  some_event.some_time = Time.utc(2000) + 1

  some_entity = SomeEntity.new
  some_projection = SomeProjection.build(entity)

  fixture(
    EntityProjection::Fixtures::Projection,
    some_projection,
    some_event
  ) do |projection|

    projection.assert_attributes_copied([
      { :example_id => :id },
      :amount
    ])

    projection.assert_transformed_and_copied(:time) { |v| Time.parse(v) }
    projection.assert_transformed_and_copied(:some_time => :other_time) { |v| Time.parse(v) }
  end
end

# Projection Fixture Facts

  • The principle concern of a handler test is the transfer of data from an event to an entity
  • The fixture tests a single event projection onto an entity, and its effects on the entity
  • The fixture can accommodate data copied between attributes that have different names on an event versus an entity
  • The fixture can accommodate data that is transformed when it is copied from an event to an entity

# EntityProjection::Fixtures::Projection Class

The Projection class is a concrete class from the EntityProjection::Fixtures library and namespace.

The EntityProjection::Fixtures::Projection class provides:

  • The instance actuator .() (or call method) that begins execution of the fixture and the actuation of its projection with the specified input event and entity
  • The assert_attributes_copied method to test that attributes are copied from the source event to the entity, including mapping between attributes that have different names on the event versus the entity
  • The assert_transformed_and_copied method to test the copying of attributes form the event and the entity where the attribute values are transformed as well as copied

# Running the Fixture

Running the test is no different than running any TestBench test.

For example, given a test file named projection.rb that uses the projection fixture, in a directory named test, the test is executed by passing the file name to the ruby executable.

ruby test/projection.rb

The test script and the fixture work together as if they are part of the same test context, preserving output nesting between the test script file and the test fixture.

# Projection Fixture Output

SomeProjection
  Apply SomeEvent to SomeEntity
    Copied
      example_id => id
      amount
    Transformed and copied
      time
    Transformed and copied
      some_time => other_time

The output below the "SomeProjection" line is from the projection fixture. The "SomeProjection" line is from the test/projection.rb test script file that is actuating the projection fixture.

# Detailed Output

In the event of any error or failed assertion, the test output will include additional detailed output that can be useful in understanding the context of the failure and the state of the fixture itself and the objects that it's testing.

The detailed output can also be printed by setting the TEST_BENCH_DETAIL environment variable to on.

TEST_BENCH_DETAIL=on ruby test/projection.rb
SomeProjection
  Projection Class: SomeProjection
  Apply SomeEvent to SomeEntity
    Event Class: SomeEvent
    Entity Class: SomeEntity
    Attributes
      example_id => id
        SomeEvent Value: "00000001-0000-4000-8000-000000000000"
        SomeEntity Value: "00000001-0000-4000-8000-000000000000"
      amount
        SomeEvent Value: 11
        SomeEntity Value: 11
    Transformed and copied
      time
        SomeEvent Value (String): "2000-01-01T00:00:00.000Z"
        SomeEntity Value (Time): 2000-01-01 00:00:00 UTC
    Transformed and copied
      some_time => other_time
        SomeEvent Value (String): "2000-01-01T00:00:00.011Z"
        SomeEntity Value (Time): 2000-01-01 00:00:00.011 UTC

# Actuating the Fixture

The fixture is executed using TestBench's fixture method.

fixture(EntityProjection::Fixtures::Projection, projection, event, &test_block)

The first argument sent to the fixture method is always the EntityProjection::Fixtures::Projection class. Subsequent arguments are the specific construction parameters of the projection fixture.

Parameters

Name Description Type
projection Projection instance used to apply the event to the entity EntityProjection
entity Object to project state into (any)
event Event to project state from Messaging::Message
test_block Block used for invoking other assertions that are part of the projection fixture's API Proc

Block Parameter

The projection_fixture argument is passed to the test_block if the block is given.

Name Description Type
projection_fixture Instance of the projection fixture that is being actuated EntityProjection::Fixtures::Projection

Block Parameter Methods

The following methods are available from the projection_fixture block parameter, and on an instance of EntityProjection::Fixtures::Projection:

  • assert_attributes_copied
  • assert_transformed_and_copied

# Testing Attribute Values Copied to the Entity

assert_attributes_copied(attribute_names=[])

The assert_attributes_copied method tests that attribute values are copied from the event being applied to the entity receiving the attribute data. By default, all attributes from the event are compared to entity attributes of the same name.

An optional list of attribute names can be passed. When the list of attribute names is passed, only those attributes will be compared. The list of attribute names can also contain maps of attribute names for comparing values when the entity attribute name is not the same as the event attribute name.

The assert_attributes_copied method uses an instance of the Schema::Fixtures::Equality fixture to attribute comparison tests.

Example

projection.assert_attributes_copied([
  { :example_id => :id },
  :amount
])

Parameters

Name Description Type
attribute_names Optional list of attribute names to compare, or maps of event attribute name to entity attribute name Array of Symbol or Hash

# Testing Individual Attribute Transformations Copied to the Entity

assert_transformed_and_copied(attribute_name, &transform)

A projection may transform or convert the event data that it's assigning to an entity. The assert_transformed_and_copied method allows an event attribute to be transformed before being compared to an entity attribute. The assertion can also accept a map to test the transformation between attributes that have a different name on the source event than on the entity.

Example

projection.assert_transformed_and_copied(:time) { |v| Time.parse(v) }
projection.assert_transformed_and_copied(:some_time => :other_time) { |v| Time.parse(v) }

Parameters

Name Description Type
attribute_name Name of the event attribute to be compared, or map of event attribute name to entity attribute name Symbol or Hash