Tips for making queries for Reports using Mango¶

Mango is a query language available to Couchdb based upon MongoDB.

General info about Mango:¶

• https://pouchdb.com/guides/mango-queries.html
• https://docs.couchdb.org/en/stable/api/database/find.html
• https://github.com/cloudant/mango

Some things to watch out for:¶

Sorting¶

Add the key you're sorting upon - in the following case, it is tangerineModifiedOn - to the index:

 await createIndex({
    index: {
      fields: [
        'type',
        'status',
        'tangerineModifiedOn'
      ]
    }
  })

$or and $ne¶

Mango abandons the indexes and does live queries, which can cause the query to fail. For $ne you can do a partial filter to improve performance.

Here is a good discussion of the issue w/ these Mango expressions: https://stackoverflow.com/a/41897093

Here is an example of this issue in Tangerine: #2367

Example lifted from the Couch doc on Partial Indexes:

To improve response times, we can create an index which excludes documents where "status": { "$ne": "archived" } at index time using the "partial_filter_selector" field:

{
  "index": {
    "partial_filter_selector": {
      "status": {
        "$ne": "Open"
      }
    },
    "fields": ["type", "status", "tangerineModifiedOn"]
  },
  "ddoc" : "type-not-open",
  "type" : "json"
}