Database-specific JSON field extraction abstraction. More...

#include <ISqlJsonXPathExtractor.h>

Inherits istd::IPolymorphic.

Inherited by imtdb::CPostgresXPathExtractorComp [virtual], and imtdb::CSqliteXPathExtractorComp [virtual].

Public Member Functions
virtual QString	ExtractXPath (const QString &jsonKey, const QString &fieldId, QMetaType::Type metaType=QMetaType::QString, const QString &tableAlias=QString()) const =0
	Extracts a JSON field using database-specific syntax.

Detailed Description

Database-specific JSON field extraction abstraction.

ISqlJsonXPathExtractor provides a database-agnostic interface for extracting values from JSON/JSONB columns in SQL databases. Different databases use different syntax for JSON operations (PostgreSQL uses operators, SQLite uses functions), and this interface abstracts those differences.

Purpose

Enables consistent JSON field access across database backends:

PostgreSQL: Uses ->, ->>, #> operators for JSONB columns
SQLite: Uses json_extract(), json_type() functions

Implementations

CPostgresXPathExtractorComp: PostgreSQL JSONB operators
CSqliteXPathExtractorComp: SQLite JSON functions

Usage Pattern

The extractor is used internally by delegates to generate database-specific JSON queries in WHERE clauses and SELECT statements:

// PostgreSQL output: metadata->>'userName'
// SQLite output: json_extract(metadata, '$.userName')
QString extracted = extractor->ExtractXPath("metadata", "userName");
 
// Use in SQL query
QString whereClause = QString("WHERE %1 = 'john'").arg(extracted);

Path Syntax

The fieldId parameter uses dot notation for nested fields:

Simple field: "userName"
Nested field: "address.city"
Array element: "tags[0]"
Deep nesting: "user.address.coordinates.latitude"

Note: Automatically handles database-specific escaping and type casting

See also: CPostgresXPathExtractorComp for PostgreSQL implementation; CSqliteXPathExtractorComp for SQLite implementation

Definition at line 62 of file ISqlJsonXPathExtractor.h.

Member Function Documentation

◆ ExtractXPath()

virtual QString imtdb::ISqlJsonXPathExtractor::ExtractXPath	(	const QString &	jsonKey,
		const QString &	fieldId,
		QMetaType::Type	metaType = `QMetaType::QString`,
		const QString &	tableAlias = `QString()`
	)		const

pure virtual

Extracts a JSON field using database-specific syntax.

Generates the appropriate SQL expression to extract a value from a JSON column based on the target database's syntax.

Parameters

jsonKey	The name of the JSON/JSONB column in the table (e.g., "metadata", "dataMetaInfo")
fieldId	The path to the field within the JSON (e.g., "userName", "address.city", "tags[0]")
metaType	Expected Qt type of the extracted value (for type casting)
tableAlias	Optional table alias prefix (e.g., "t1" for "t1.metadata")

Returns: SQL expression string for extracting the JSON field

Examples:

PostgreSQL:

// Extract string value
QString expr = extractor->ExtractXPath("metadata", "userName");
// Returns: metadata->>'userName'
 
// Extract nested value
QString expr = extractor->ExtractXPath("metadata", "address.city");
// Returns: metadata#>>'{address,city}'
 
// Extract with table alias
QString expr = extractor->ExtractXPath("metadata", "age", QMetaType::Int, "users");
// Returns: (users.metadata->>'age')::integer

SQLite:

// Extract string value
QString expr = extractor->ExtractXPath("metadata", "userName");
// Returns: json_extract(metadata, '$.userName')
 
// Extract nested value
QString expr = extractor->ExtractXPath("metadata", "address.city");
// Returns: json_extract(metadata, '$.address.city')
 
// Extract with table alias
QString expr = extractor->ExtractXPath("metadata", "age", QMetaType::Int, "users");
// Returns: CAST(json_extract(users.metadata, '$.age') AS INTEGER)

Note: The metaType parameter enables proper type casting (important for numeric comparisons); Array access syntax varies by database and is handled internally; Nested fields are properly escaped to prevent SQL injection

See also: CPostgresXPathExtractorComp, CSqliteXPathExtractorComp