When to use nested results in MongoDB aggregation?
Spring Data MongoDB’s nested method is very useful for creating well-structured output from our aggregation queries. Doing this step during aggregation helps us avoid having java code to post-process the result.
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How is the aggregation pipeline modeled in MongoDB?
Aggregation pipeline¶. MongoDB’s aggregation framework is based on the concept of data processing pipelines. The documents go into a multi-stage pipeline that transforms the documents into an aggregate output.
Is there a way to add documents in MongoDB?
MongoDB also provides db.collection.estimatedDocumentCount(), db.collection.count(), and db.collection.distinct(). All of these operations aggregate documents from a single collection. While these operations provide simple access to common aggregation processes, they lack the flexibility and capabilities of an aggregation pipeline.
How to conditionally exclude a field in MongoDB?
As of MongoDB 3.6, you can use the REMOVE variable in aggregation expressions to conditionally remove a field. For an example, see Conditional Exclusion Fields. MongoDB also provides $addFields to add new fields to documents.
How does $groupstage work in MongoDB?
The _id field of each output document contains the unique group by value. Output documents can also contain calculated fields that contain the values of some accumulator expression. $group does not sort your output documents. The $ group stage has the following prototype form:
How to calculate the sum of multiple fields in MongoDB?
It has its own operator, we can get the current document field using the $ symbol + field name. To first understand MongoDB’s group by multiple fields, let’s take a look at a list of all the operators that can be used on $group: $sum – Returns the sum of all numeric fields. $avg – Calculates the average between numeric fields.
What is the memory limit for aggregation in MongoDB?
Db.collection.aggregate() uses native MongoDB operations to efficiently aggregate and support operations like GroupBy in SQL without having to write custom JS routines. Each phase of the pipeline limits 100 MB of memory. If a single-node pipeline exceeds the limit, MongoDB throws an error.