feat(firestore): pipeline subequeries

[daniel-sanche]

Add support for Pipeline subqueries, allowing users to perform complex data transformation by embedding a full pipeline inside another

New Stages:

• define
• subcollection

New Expressions:

• current_document
• variable
• get_field

New classes:

• SubPipeline, which represents a pipeline without an associated client, which can not be executed

[gemini-code-assist]

Code Review

This pull request introduces support for subqueries and variables within Firestore pipelines. Key additions include methods to convert pipelines into array or scalar expressions, a define stage for binding variables, and a subcollection source for relative path queries. New expression types like Variable and CurrentDocument were also implemented alongside corresponding system tests. Feedback was provided to add a type hint to the _PipelineValueExpression constructor to improve type safety and code clarity.

[bhshkh]

Is this being named Subpipeline to avoid name collision ? Is there an existing Subcollection class ?

Java calls this stage Subcollection https://github.com/googleapis/java-firestore/blob/0c8188520dfbada0d3fef0719e4f95fc231306be/google-cloud-firestore/src/main/java/com/google/cloud/firestore/PipelineSource.java#L214-L238

[daniel-sanche]

This isn't a stage, it's a pipeline subclass. I think this is probably Python-specific, because of the weird way we have to handle async/sync code

In Python, we have 3 classes:

• Pipeline to handle sync clients
• AsyncPipeline to handle async clients
• _BasePipeline as a private class to handle common logic between the two.

Sub-pipelines presents a new use-case, where neither client is be attached, but it needs to be exposed to end-users

In Java and other implementations, it's much simpler, because there is a single pipeline class, and they just raise an exception if you try to use it to execute on a sub-pipeline.

[bhshkh]

In java, this is static https://github.com/googleapis/java-firestore/blob/0c8188520dfbada0d3fef0719e4f95fc231306be/google-cloud-firestore/src/main/java/com/google/cloud/firestore/PipelineSource.java#L214-L238

Not sure what would be the equivalent in Python

[daniel-sanche]

Good catch, I'll fix that