如何确定JSON字段的数据类型？MongoDB中能否查询该类型？

Great questions! Let's break this down step by step.

1. Determining a Field's Data Type in JSON #

JSON has a small set of native data types, so you can spot a field's type just by looking at its value format:

• String: Enclosed in double quotes (like "Fri Aug 04 05:42:03 +0000 2017" from your example)
• Number: Numeric values without quotes (e.g., 893346273255866400 for your id field)
• Boolean: true or false (no quotes)
• Array: Wrapped in square brackets []
• Object: Wrapped in curly braces {}
• Null: Explicit null value

For your sample document, the created_at value is wrapped in double quotes, so it’s definitely a string type.

2. Checking Field Data Types in MongoDB #

Absolutely, you can verify the data type of a specific field in MongoDB documents—here are two simple, reliable methods:

Method 1: Use the $type Query Operator #

MongoDB’s $type operator lets you query documents where a field matches a specific data type. You can use either the type name (e.g., "string") or its corresponding numeric code (e.g., 2 for strings).

To confirm your created_at field is a string:

// Query using the type name
db.your_collection.find({ "created_at": { $type: "string" } })

// Or use the numeric type code
db.your_collection.find({ "created_at": { $type: 2 } })

If your sample document is returned, that confirms created_at is stored as a string.

Method 2: Inspect Individual Document Field Types #

Grab a single document and use JavaScript’s typeof operator to check the field’s type directly in the MongoDB shell:

// Fetch the document (adjust the query to match your data)
const doc = db.your_collection.findOne({ id: 893346273255866400 })

// Check the type of the created_at field
typeof doc.created_at // Will return "string"

Bonus: Working with String Dates in Queries #

Since your created_at is a string-formatted date, if you want to run date-based queries (like filtering by date range), you can convert it to a MongoDB Date type on the fly using $toDate:

// Find documents where created_at is on or after August 1, 2017
db.your_collection.find({
  $expr: {
    $gte: [
      { $toDate: "$created_at" },
      ISODate("2017-08-01T00:00:00Z")
    ]
  }
})

内容的提问来源于stack exchange，提问作者codex