MongoDB Cursor Usage: The Correct Way to Iterate Over Query Results
MongoDB cursors are "navigation tools" for query results, with core features of **lazy execution** (query triggered only during traversal) and iterator properties (returning one piece of data at a time, suitable for large datasets). Cursors are obtained via the `find()` method, supporting parameters like conditions, sorting, and limiting, e.g., `find(query, fieldProjection).sort().limit()`. There are three common traversal methods: `forEach()` (simple, for small datasets), `toArray()` (loads all data into memory, only suitable for small datasets, disabled for large datasets), and `while` loop with `next()` (manual control, suitable for large datasets). Key notes: Avoid `toArray()` for large datasets to prevent memory overflow; cursors have a default 10-minute timeout, adjustable via `maxTimeMS`; data consistency uses snapshot reads; avoid `skip()` for pagination, use `_id` anchor positioning instead; for large datasets, iterate in batches and control `batchSize`. In summary: Use `forEach()` for small data, `while+next()` for large data, avoid `toArray()` and `skip()`. Mastering these ensures efficient and secure data traversal.
Read MoreMongoDB Sorting and Projection: Making Query Results "Attractive and Useful"
In MongoDB, sorting and projection can optimize query results. Sorting is implemented using `find().sort({ field: 1/-1 })`, where `1` denotes ascending order and `-1` denotes descending order. Multi-field sorting is supported (e.g., `sort({ age: 1, score: -1 })`). Projection controls returned fields with `find(condition, { field: 1/0 })`, where `1` retains the field and `0` excludes it; `_id: 0` must be explicitly set to exclude the default `_id` field. These can be combined, such as querying "students over 17 years old, sorted by age ascending, only showing name and age" to get ordered and concise results. Key points: sorting direction is 1/-1, projection requires manual exclusion of `_id`, and flexible combination is applicable in various scenarios.
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