feat(firestore): literals pipeline stage

[Linchin]

Succeeding googleapis/python-firestore#1170 for the monorepo migration.

[gemini-code-assist]

Summary of Changes

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This pull request enhances the Firestore client library by introducing a new literals pipeline stage. This stage provides a mechanism to inject a fixed set of predefined document objects directly into a pipeline, offering greater flexibility for testing individual pipeline stages and serving as initial inputs for complex operations like join conditions. The change includes the necessary class implementation, method integration into the base pipeline, and corresponding test coverage.

Highlights

• New literals pipeline stage: Introduced a new literals method to the _BasePipeline class, allowing users to define documents from a fixed set of predefined objects. This stage is particularly useful for testing other pipeline stages in isolation or as inputs for join conditions.
• Implementation of Literals class: Added the Literals class in pipeline_stages.py to represent the new pipeline stage, including its constructor and methods for protobuf serialization.
• Comprehensive testing: Included new system-level and unit tests to ensure the correct functionality, behavior, and integration of the literals pipeline stage.

Changelog

• packages/google-cloud-firestore/google/cloud/firestore_v1/base_pipeline.py
  • Added the literals method to _BasePipeline, enabling the definition of documents from a fixed set.
  • Included detailed docstrings with examples and behavior descriptions for the new literals method.
• packages/google-cloud-firestore/google/cloud/firestore_v1/pipeline_stages.py
  • Introduced the Literals class, inheriting from Stage, to represent the new pipeline stage.
  • Implemented the __init__ and _pb_args methods for the Literals class to handle document arguments and their protobuf serialization.
• packages/google-cloud-firestore/tests/system/pipeline_e2e/general.yaml
  • Appended a new system test case for the literals pipeline stage, demonstrating its usage with a sample document and asserting the expected results and protobuf structure.
• packages/google-cloud-firestore/tests/unit/v1/test_pipeline.py
  • Updated the test_pipeline_methods parameterized test to include the new literals stage, ensuring it can be correctly appended to a pipeline.
• packages/google-cloud-firestore/tests/unit/v1/test_pipeline_stages.py
  • Added a new test class TestLiterals with unit tests for the Literals stage's constructor, representation (repr), and protobuf serialization (_to_pb).

Activity

• No human activity has been recorded on this pull request yet.

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[gemini-code-assist]

Code Review

This pull request introduces a new literals pipeline stage, which allows specifying a fixed set of documents as the starting point of a pipeline. The implementation includes the literals method on the pipeline builder, the Literals stage class, and corresponding unit and end-to-end tests. My review focuses on improving the clarity and correctness of the type hints and docstrings for the new functionality. I've suggested changes to make the API easier to understand and use correctly.

[gemini-code-assist]

The type hint for *documents is incomplete. It should include dict as documents are often passed as dictionaries, which is not covered by str | Selectable.

Suggested change

	def literals(self, *documents: str | Selectable) -> "_BasePipeline":
	def literals(self, *documents: dict | str | Selectable) -> "_BasePipeline":

[gemini-code-assist]

The Args section of the docstring is misleading. It describes a single argument, but the method accepts *documents. It also incorrectly states that a str is treated as a field path; the implementation treats it as a string literal. To use a field path, field() should be used explicitly.

Suggested change

	documents: A `str` or `Selectable` expression. If a `str`, it's
	treated as a field path to an array of documents.
	If a `Selectable`, it's usually a `Constant`
	containing an array of documents (as dictionaries).
	documents: One or more documents to be returned by this stage.
	Each can be a `dict`, a `str` literal, or a
	`Selectable` expression (e.g. `field("path")` or
	`Constant.of(...)`).

[gemini-code-assist]

The type hint for *documents is incomplete. It should include dict as documents are often passed as dictionaries, which is not covered by str | Selectable.

Suggested change

	def __init__(self, *documents: str | Selectable):
	def __init__(self, *documents: dict | str | Selectable):