Add type-safety in operators

[System-Glitch]

fixes #7

Type safety

Added type-safety. The operators now generate valid SQL matching the column's data type. The data type is determined by the new struct tag fieldType. If not provided, the type will be determined from GORM's data type. If GORM's data type is a custom type or a type that is not directly supported by this library, the type will fall back to - (unsupported).

See the updated README for more details.

Breaking change

This change breaks custom operators: the function signature changed.

//Before
func(tx *gorm.DB, filter *filter.Filter, column string, dataType schema.DataType) *gorm.DB

// After
func(tx *gorm.DB, f *filter.Filter, column string, dataType filter.DataType) *gorm.DB

You may need to update your models by adding the new fieldType struct tag on non-native types (such as arrays and enum types). Structures implementing driver.Valuer should work without needing for a change.

type MyModel struct {
	ID         uint
	SomeArray  []string     `gorm:"type:VARCHAR(255)[]" filterType:"text[]"`
	SomeEnum   MyCustomEnum `gorm:"type:my_custom_enum" filterType:"enum"`
}

Fields having a custom database type set (gorm:"type:DOUBLE PRECISION" for example) should work too as the type detection is based on GORMDataType and not on the actual DataType.

Types from the guregu/null library should work without having to add the fieldType tag since they implement driver.Valuer.

You may also need to update computed columns that would return a type that doesn't exactly match the struct field. This is common when working with JSON nested fields. Database engines usually always return text types from JSON operations, which is incompatible with numbers.

// This example is compatible with PostgreSQL.
// JSON processing may be different if you are using another database engine. 
type MyModel struct {
	ID            uint
	JSONColumn    datatypes.JSON
	SomeJSONField null.Int `gorm:"->;-:migration" computed:"(~~~ct~~~.json_column->>'fieldName')::int"`
}

[coveralls]

Pull Request Test Coverage Report for Build 4636597426

• 192 of 192 (100.0%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 99.241%

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Totals
Change from base Build 3386715886:	0.2%
Covered Lines:	915
Relevant Lines:	922

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💛 - Coveralls