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`$expr` lets you use **expressions and operations between fields within a single document** while filtering. This isn't possible with plain filters, they only compare a field against a fixed value, while `$expr` can compare **field to field**, and run operations like `$gt`, `$add`, `$subtract`, and more.

### When `$expr` is needed, in plain terms

It's used when a condition can't be expressed through a plain filter. For example:

- you need to compare one field against another
- you need to apply a calculation right inside the filter (addition, subtraction, etc.)
- you need to use aggregation operators inside the query

### An example: comparing field to field

**The task:** find documents where `spent > limit`
(a plain filter can't do this, it only compares against a fixed value)

```js
db.users.find({
  $expr: { $gt: ["$spent", "$limit"] }
})
```

### An example: comparing against a calculation

Find products whose price is higher than a discounted version `(price * 0.9)`:

```js
db.products.find({
  $expr: { $gt: ["$price", { $multiply: ["$discountPrice", 1.1] }] }
})
```

### The summary for a junior developer:

| Filter | When to use it |
|---|---|
| A plain `{ field: value }` | simple comparisons against constants |
| `$expr` | comparing **field to field** or **complex expressions** |

**The takeaway:** `$expr` is needed when the filtering logic depends on calculations or comparisons between fields of the same document.

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