# Body - Fields { #body-fields }

The same way you can declare additional validation and metadata in *path operation function* parameters with `Query`, `Path` and `Body`, you can declare validation and metadata inside of Pydantic models using Pydantic's `Field`.

## Import `Field` { #import-field }

First, you have to import it:

{* ../../docs_src/body_fields/tutorial001_an_py310.py hl[4] *}


/// warning

        List<String> readings = Arrays.asList("reading1", "reading2");
        List<String> fields = Arrays.asList("field1", "field2", "field3");
        List<String> tags = Arrays.asList("tag1", "tag2");
        List<String> roles = Arrays.asList("role1", "role2", "role3");

        elevateWord = new ElevateWord(word, boost, readings, fields, tags, roles);
        SuggestItem suggestItem = elevateWord.toSuggestItem();

    @Test
    public void testGetKeywordsWithEmptyField() {
        // Test with empty field array
        String query = "test query";
        String[] fields = {};
        List<String> keywords = SuggestUtil.getKeywords(query, fields);
        assertNotNull(keywords);
        assertEquals(0, keywords.size()); // No keywords because no fields match
    }

    @Test
    public void testGetKeywordsWithComplexQuery() {

# Dependencies with yield { #dependencies-with-yield }

FastAPI supports dependencies that do some <abbr title='sometimes also called "exit code", "cleanup code", "teardown code", "closing code", "context manager exit code", etc.'>extra steps after finishing</abbr>.

To do this, use `yield` instead of `return`, and write the extra steps (code) after.

/// tip

Make sure to use `yield` one single time per dependency.

///

/// note | Technical Details

// JSON utilities with proper escaping
String escaped = JsonUtil.escape("Hello \"World\" with special chars");
String unescaped = JsonUtil.unescape(escaped);

// Text tokenization
Tokenizer tokenizer = new Tokenizer("field1,field2,field3", ",");
while (tokenizer.hasMoreTokens()) {
    String token = tokenizer.nextToken();
    // Process each token
}

// Decimal formatting

// whitespace characters, as defined by [unicode.IsSpace].
// The iterator yields the same subslices that would be returned by [Fields](s),
// but without constructing a new slice containing the subslices.
func FieldsSeq(s []byte) iter.Seq[[]byte] {
	return func(yield func([]byte) bool) {
		start := -1
		for i := 0; i < len(s); {
			size := 1
			r := rune(s[i])

        item.setReadings(newReadings);
        assertArrayEquals(newReadings, item.getReadings());

        // Test fields
        String[] fields = { "field1", "field2" };
        item.setFields(fields);
        assertArrayEquals(fields, item.getFields());

        // Test tags
        String[] tags = { "tag1", "tag2" };
        item.setTags(tags);

The `HeroUpdate` *data model* is somewhat special, it has **all the same fields** that would be needed to create a new hero, but all the fields are **optional** (they all have a default value). This way, when you update a hero, you can send just the fields that you want to update.

Because all the **fields actually change** (the type now includes `None` and they now have a default value of `None`), we need to **re-declare** them.

            parser.setDefaultOperator(StandardQueryConfigHandler.Operator.AND);

            // Parse with the first field if available, otherwise use "default"
            String defaultField = fields != null && fields.length > 0 ? fields[0] : "default";
            termQueryList = getTermQueryList(parser.parse(q, defaultField), fields);
        } catch (final Exception e) {
            return keywords;
        }

<div class="screenshot">
<img src="/img/tutorial/request-form-models/image01.png">
</div>

## Forbid Extra Form Fields { #forbid-extra-form-fields }

In some special use cases (probably not very common), you might want to **restrict** the form fields to only those declared in the Pydantic model. And **forbid** any **extra** fields.

/// note

This is supported since FastAPI version `0.114.0`. 🤓

///