ActiveModel::Serialization is simple in that it recursively calls
`as_json` on each object to serialize everything. However, for a model
like a Project, this can generate a query for every single association,
which can add up to tens of thousands of queries and lead to memory
bloat.
To improve this, we can do several things:
1. We use `tree:` and `preload:` to automatically generate
a list of all preloads that could be used to serialize
objects in bulk.
2. We observe that a single project has many issues, merge requests,
etc. Instead of serializing everything at once, which could lead to
database timeouts and high memory usage, we take each top-level
association and serialize the data in batches.
For example, we serialize the first 100 issues and preload all of
their associated events, notes, etc. before moving onto the next
batch. When we're done, we serialize merge requests in the same way.
We repeat this pattern for the remaining associations specified in
import_export.yml.
This brings a significant refactor to how
we handle `import_export.yml`, merge it with EE
and how we handle that for reader and saver.
This is meant to simplify the code, and remove
a ton of conditions to handle different models
of the structure.
This is also meant to prepare the structure
to extend it much easier, like adding `preload:`
or additional object types when needed.
This does not change the behavior of import/export,
rather unifies and simplifies the current implementation.
Kamil Trzciński