Document sql_expr (#3643)

Author: andreyaksenov

doc/code_snippets/test/constraints/constraint_sql_expr_test.lua

@@ -0,0 +1,62 @@
+local fio = require('fio')
+local server = require('luatest.server')
+local t = require('luatest')
+local g = t.group()
+g.before_each(function(cg)
+    cg.server = server:new {
+        box_cfg = {},
+        workdir = fio.cwd() .. '/tmp'
+    }
+    cg.server:start()
+end)
+
+g.after_each(function(cg)
+    cg.server:stop()
+    cg.server:drop()
+end)
+
+g.test_constraints = function(cg)
+    cg.server:exec(function()
+        -- create_constraint_start
+        box.schema.func.create('check_person', {
+            language = 'SQL_EXPR',
+            is_deterministic = true,
+            body = [["age" > 21 AND "name" != 'Admin']]
+        })
+        -- create_constraint_end
+
+        -- configure_space_start
+        local customers = box.schema.space.create('customers', { constraint = 'check_person' })
+        customers:format({
+            { name = 'id', type = 'number' },
+            { name = 'name', type = 'string' },
+            { name = 'age', type = 'number' },
+        })
+        customers:create_index('primary', { parts = { 1 } })
+        -- configure_space_end
+
+        -- insert_ok_start
+        customers:insert { 1, "Alice", 30 }
+        -- insert_ok_end
+
+        local _, age_err = pcall(function()
+            -- insert_age_error_start
+            customers:insert { 2, "Bob", 18 }
+            -- error: Check constraint 'check_person' failed for tuple
+            -- insert_age_error_end
+        end)
+
+        local _, name_err = pcall(function()
+            -- insert_name_error_start
+            customers:insert { 3, "Admin", 25 }
+            -- error: Check constraint 'check_person' failed for tuple
+            -- insert_name_error_end
+        end)
+
+        -- Tests --
+        t.assert_equals(customers:count(), 1)
+        t.assert_equals(customers:get(1), { 1, "Alice", 30 })
+        t.assert_equals(age_err:unpack().message, 'Check constraint \'check_person\' failed for tuple')
+        t.assert_equals(name_err:unpack().message, 'Check constraint \'check_person\' failed for tuple')
+    end)
+end

doc/concepts/data_model/value_store.rst

@@ -598,11 +598,11 @@ a wider range of limitations.
 Constraint functions
 ~~~~~~~~~~~~~~~~~~~~
 
-Constraints use stored Lua functions, which must return ``true`` when the constraint
+Constraints use stored Lua functions or :ref:`SQL expressions <sql_expressions>`, which must return ``true`` when the constraint
 is satisfied. Other return values (including ``nil``) and exceptions make the
 check fail and prevent tuple insertion or modification.
 
-To create a constraint function, use :ref:`func.create with function body <box_schema-func_create_with-body>`.
+To create a constraint function, call :ref:`box.schema.func.create() <box_schema-func_create>` with the function definition specified in the ``body`` attribute.
 
 Constraint functions take two parameters: