链式方法

GORM’s method chaining feature allows for a smooth and fluent style of coding. Here’s an example:

db.Where("name = ?", "jinzhu").Where("age = ?", 18).First(&user)

Method Categories

GORM organizes methods into three primary categories: Chain Methods, Finisher Methods, and New Session Methods.

Chain Methods

Chain methods are used to modify or append Clauses to the current Statement. Some common chain methods include:

• Where
• Select
• Omit
• Joins
• Scopes
• Preload
• Raw (Note: Raw cannot be used in conjunction with other chainable methods to build SQL)

For a comprehensive list, visit GORM Chainable API. Also, the SQL Builder documentation offers more details about Clauses.

Finisher Methods

Finisher methods are immediate, executing registered callbacks that generate and run SQL commands. This category includes methods:

• Create
• First
• Find
• Take
• Save
• Update
• Delete
• Scan
• Row
• Rows

For the full list, refer to GORM Finisher API.

New Session Methods

GORM defines methods like Session, WithContext, and Debug as New Session Methods, which are essential for creating shareable and reusable *gorm.DB instances. For more details, see Session documentation.

Reusability and Safety

A critical aspect of GORM is understanding when a *gorm.DB instance is safe to reuse. Following a Chain Method or Finisher Method, GORM returns an initialized *gorm.DB instance. This instance is not safe for reuse as it may carry over conditions from previous operations, potentially leading to contaminated SQL queries. For example:

Example of Unsafe Reuse

queryDB := DB.Where("name = ?", "jinzhu")

// First query
queryDB.Where("age > ?", 10).First(&user)
// SQL: SELECT * FROM users WHERE name = "jinzhu" AND age > 10

// Second query with unintended compounded condition
queryDB.Where("age > ?", 20).First(&user2)
// SQL: SELECT * FROM users WHERE name = "jinzhu" AND age > 10 AND age > 20

Example of Safe Reuse

To safely reuse a *gorm.DB instance, use a New Session Method:

queryDB := DB.Where("name = ?", "jinzhu").Session(&gorm.Session{})

// First query
queryDB.Where("age > ?", 10).First(&user)
// SQL: SELECT * FROM users WHERE name = "jinzhu" AND age > 10

// Second query, safely isolated
queryDB.Where("age > ?", 20).First(&user2)
// SQL: SELECT * FROM users WHERE name = "jinzhu" AND age > 20

In this scenario, using Session(&gorm.Session{}) ensures that each query starts with a fresh context, preventing the pollution of SQL queries with conditions from previous operations. This is crucial for maintaining the integrity and accuracy of your database interactions.

Examples for Clarity

Let’s clarify with a few examples:

• Example 1: Safe Instance Reuse

db, err := gorm.Open(sqlite.Open("test.db"), &gorm.Config{})
// 'db' is a newly initialized `*gorm.DB`, which is safe to reuse.

db.Where("name = ?", "jinzhu").Where("age = ?", 18).Find(&users)
// The first `Where("name = ?", "jinzhu")` call is a chain method that initializes a `*gorm.DB` instance, or `*gorm.Statement`.
// The second `Where("age = ?", 18)` call adds a new condition to the existing `*gorm.Statement`.
// `Find(&users)` is a finisher method, executing registered Query Callbacks, generating and running:
// SELECT * FROM users WHERE name = 'jinzhu' AND age = 18;

db.Where("name = ?", "jinzhu2").Where("age = ?", 20).Find(&users)
// Here, `Where("name = ?", "jinzhu2")` starts a new chain, creating a fresh `*gorm.Statement`.
// `Where("age = ?", 20)` adds to this new statement.
// `Find(&users)` again finalizes the query, executing and generating:
// SELECT * FROM users WHERE name = 'jinzhu2' AND age = 20;

db.Find(&users)
// Directly calling `Find(&users)` without any `Where` starts a new chain and executes:
// SELECT * FROM users;

In this example, each chain of method calls is independent, ensuring clean, non-polluted SQL queries.

• (Bad) Example 2: Unsafe Instance Reuse

db, err := gorm.Open(sqlite.Open("test.db"), &gorm.Config{})
// 'db' is a newly initialized *gorm.DB, safe for initial reuse.

tx := db.Where("name = ?", "jinzhu")
// `Where("name = ?", "jinzhu")` initializes a `*gorm.Statement` instance, which should not be reused across different logical operations.

// Good case
tx.Where("age = ?", 18).Find(&users)
// Reuses 'tx' correctly for a single logical operation, executing:
// SELECT * FROM users WHERE name = 'jinzhu' AND age = 18

// Bad case
tx.Where("age = ?", 28).Find(&users)
// Incorrectly reuses 'tx', compounding conditions and leading to a polluted query:
// SELECT * FROM users WHERE name = 'jinzhu' AND age = 18 AND age = 28;

In this bad example, reusing the tx variable leads to compounded conditions, which is generally not desirable.

• Example 3: Safe Reuse with New Session Methods

db, err := gorm.Open(sqlite.Open("test.db"), &gorm.Config{})
// 'db' is a newly initialized *gorm.DB, safe to reuse.

tx := db.Where("name = ?", "jinzhu").Session(&gorm.Session{})
tx := db.Where("name = ?", "jinzhu").WithContext(context.Background())
tx := db.Where("name = ?", "jinzhu").Debug()
// `Session`, `WithContext`, `Debug` methods return a `*gorm.DB` instance marked as safe for reuse. They base a newly initialized `*gorm.Statement` on the current conditions.

// Good case
tx.Where("age = ?", 18).Find(&users)
// SELECT * FROM users WHERE name = 'jinzhu' AND age = 18

// Good case
tx.Where("age = ?", 28).Find(&users)
// SELECT * FROM users WHERE name = 'jinzhu' AND age = 28;

In this example, using New Session Methods Session, WithContext, Debug correctly initializes a *gorm.DB instance for each logical operation, preventing condition pollution and ensuring each query is distinct and based on the specific conditions provided.

Overall, these examples illustrate the importance of understanding GORM’s behavior with respect to method chaining and instance management to ensure accurate and efficient database querying.