New Features 🎉

• Implement $group stage _id expression by @chilagrow in #3138
• Implement $expr evaluation query operator by @chilagrow in #3163

Fixed Bugs 🐛

• Do not return immutable _id error from findAndModify for upserting same _id by @chilagrow in #3171

Enhancements 🛠

• Cache SQLite tables metadata by @AlekSi in #3124
• Use lock for SQLite metadata by @AlekSi in #3146

Other Changes 🤖

• Make SQLite integration tests pass by @noisersup in #3150, #3151, #3152, #3159, #3173, #3174, #3182, #3184, #3185
• Expect some aggregate and insert tests to fail for SQLite by @AlekSi in #3147
• Bump deps by @AlekSi in #3142
• Improve MongoDB/FerretDB error checking in tests by @AlekSi in #3143
• Bump deps, including Go by @AlekSi in #3160
• Simplify tests a bit by @AlekSi in #3149
• Use one implementation for finding path values by @chilagrow in #3087
• Add tests for $$ROOT aggregation expression variable by @chilagrow in #3180
• Add tests for $$ROOT by @chilagrow in #3187

All closed issues and pull requests.
All commits.