MongoDB Learning Notes Aggregation pipeline practical implementation INNER JOIN

Aggregation pipeline to achieve inner join:

LEFTT Collection:

Comments Collection:

Now we're going to implement inner join with aggregation.

Db.comments.aggregate ([{$lookup: {from:"LEFTT", Localfield:"timestamp", Foreignfield:" Timestamp "" Timestampcol"" $timestampCol "},{$project: {" Ratingeq ": {" $eq ": [" $rating "," $ Timestampcol.rating "]}," timestamp ": 1," rating ": 1,  " Timestampcol.timestamp ": 1}},{$match: {" Ratingeq " :true}}])

Isn't it amazing?

Step parsing:

1. $lookup: Implements timestamp equivalent connection, equal LEFTT result is placed in Timestampcol collection
2. $unwind: Implemented to flatten the LEFTT array, there are multiple
3. $project: Implementation of the second equivalent connection rating, and placed in the Ratingeq field
4. $match: Take only Ratingeq is true, indicating that the second rating field is equal

MongoDB Learning Note Aggregation Pipeline actual combat implementation inner JOIN