Fix exists queries on nested flat_object fields throws exception

[kkewwei]

Description

[Describe what this change achieves]

Related Issues

Resolves #16795

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

❌ Gradle check result for 5763a44: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

✅ Gradle check result for 7d38155: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 72.29%. Comparing base (2794655) to head (b7d7cf9).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
...opensearch/index/mapper/FlatObjectFieldMapper.java	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #16803      +/-   ##
============================================
+ Coverage     72.19%   72.29%   +0.09%     
- Complexity    65304    65323      +19     
============================================
  Files          5301     5301              
  Lines        303774   303776       +2     
  Branches      44034    44035       +1     
============================================
+ Hits         219323   219626     +303     
+ Misses        66458    66078     -380     
- Partials      17993    18072      +79     

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[github-actions]

❌ Gradle check result for d955649: FAILURE

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[github-actions]

❌ Gradle check result for 3ec0dfe: FAILURE

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[github-actions]

❌ Gradle check result for 56e6036: FAILURE

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[github-actions]

❌ Gradle check result for 0ea853d: FAILURE

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[github-actions]

✅ Gradle check result for b7d7cf9: SUCCESS

[kkewwei]

@msfroh, Could you please help review the pull request at your convenience.

[msfroh]

Wow... thanks for catching that! It's a nice small fix.

[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/OpenSearch/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/OpenSearch/backport-2.x
# Create a new branch
git switch --create backport/backport-16803-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 c6dfc65ea0bccc9cfe66bc4248d09b42d7430d0e
# Push it to GitHub
git push --set-upstream origin backport/backport-16803-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/OpenSearch/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-16803-to-2.x.

[msfroh]

Hmm... sorry @kkewwei -- the automatic backport failed. It's most likely a conflict on the changelog again ☹️

[kkewwei]

Hmm... sorry @kkewwei -- the automatic backport failed. It's most likely a conflict on the changelog again ☹️

Some of the previous commits are not backported, I will complete them and then backport this one.