feat: add thenRef method in eb.case (#1531)

Co-authored-by: igalklebanov <igalklebanov@gmail.com>

Author: ericsodev

src/query-builder/case-builder.ts

@@ -93,6 +93,29 @@ export class CaseThenBuilder<DB, TB extends keyof DB, W, O> {
       ),
     })
   }
+
+  /**
+   * Adds a `then` clause to the `case` statement where the value is a reference to a column.
+   *
+   * A `thenRef` call can be followed by {@link Whenable.when}, {@link CaseWhenBuilder.else},
+   * {@link CaseWhenBuilder.end} or {@link CaseWhenBuilder.endCase} call.
+   */
+  thenRef<RE extends ReferenceExpression<DB, TB>>(
+    expression: RE,
+  ): CaseWhenBuilder<
+    DB,
+    TB,
+    W,
+    O | ExtractTypeFromReferenceExpression<DB, TB, RE>
+  > {
+    return new CaseWhenBuilder({
+      ...this.#props,
+      node: CaseNode.cloneWithThen(
+        this.#props.node,
+        parseReferenceExpression(expression),
+      ),
+    })
+  }
 }
 
 export class CaseWhenBuilder<DB, TB extends keyof DB, W, O>
@@ -196,7 +219,7 @@ interface Whenable<DB, TB extends keyof DB, W, O> {
   /**
    * Adds a `when` clause to the case statement.
    *
-   * A `when` call must be followed by a {@link CaseThenBuilder.then} call.
+   * A `when` call must be followed by either a {@link CaseThenBuilder.then} or {@link CaseThenBuilder.thenRef} call.
    */
   when<
     RE extends ReferenceExpression<DB, TB>,

test/node/src/case.test.ts

@@ -60,6 +60,40 @@ for (const dialect of DIALECTS) {
       await query.execute()
     })
 
+    it('should execute a query with a case...when...thenRef...end operator', async () => {
+      const query = ctx.db
+        .selectFrom('person')
+        .select((eb) =>
+          eb
+            .case()
+            .when('gender', '=', 'male')
+            .thenRef('first_name')
+            .end()
+            .as('title'),
+        )
+
+      testSql(query, dialect, {
+        postgres: {
+          sql: `select case when "gender" = $1 then "first_name" end as "title" from "person"`,
+          parameters: ['male'],
+        },
+        mysql: {
+          sql: 'select case when `gender` = ? then `first_name` end as `title` from `person`',
+          parameters: ['male'],
+        },
+        mssql: {
+          sql: `select case when "gender" = @1 then "first_name" end as "title" from "person"`,
+          parameters: ['male'],
+        },
+        sqlite: {
+          sql: `select case when "gender" = ? then "first_name" end as "title" from "person"`,
+          parameters: ['male'],
+        },
+      })
+
+      await query.execute()
+    })
+
     it('should execute a query with a case...value...when...then...end operator', async () => {
       const query = ctx.db
         .selectFrom('person')

test/typings/test-d/case.test-d.ts

@@ -69,16 +69,28 @@ async function testCase(eb: ExpressionBuilder<Database, 'person'>) {
       .end(),
   )
 
-  // references
+  // eb.refs
   expectType<ExpressionWrapper<Database, 'person', string | number>>(
     eb
       .case()
-      .when('gender', '=', 'male')
+      .when('first_name', '=', eb.ref('last_name'))
       .then(eb.ref('first_name'))
       .else(eb.ref('age'))
       .end(),
   )
 
+  // refs
+  expectType<ExpressionWrapper<Database, 'person', string | number | null>>(
+    eb
+      .case()
+      .when('first_name', '=', eb.ref('last_name')) // TODO: whenRef
+      .thenRef('first_name')
+      .when('deleted_at', 'is not', null)
+      .thenRef('age')
+      // .elseRef('age')
+      .end(),
+  )
+
   // expressions
   expectType<ExpressionWrapper<Database, 'person', `Mr. ${string}` | null>>(
     eb