fix(core): Keep order of nested relations during hydration (#2864)

[jnugh]

Description

Relations could get out of sync during entity hydration. See #2864 for a detailed description and example.

I moved mergeDeep out of the service to make testing easier. I think adding an e2e Test for this does not make sense as it would be very hard to control the result order of the hydration database query. So I decided to unit test mergeDeep instead.

The only actual change is:

                if (Array.isArray(value)) {
                    (a as any)[key] = value.map((v, index) =>
                        this.mergeDeep(a?.[key]?.[index], b[key][index]),
                    );
                } else if (isObject(value)) {
                    (a as any)[key] = this.mergeDeep(a?.[key], b[key]);
                }

changed to:

            if (Array.isArray(value) || isObject(value)) {
                (a as any)[key] = mergeDeep(a?.[key], b[key]);
            }

I don't see a case where calling mergeDeep would not work because mergeDeep still executes the same logic inside the recursion by iterating using for (const [key, value] of Object.entries(b)) and assigning the values using (a as any)[key] = b[key];, event if a and b do not contain entities.

Breaking changes

None

Checklist

📌 Always:

• I have set a clear title
• My PR is small and contains a single feature
• I have checked my own PR

👍 Most of the time:

• I have added or updated test cases
• I have updated the README if needed

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[michaelbromley]

Thanks for the detailed write-up & fix! 👍