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diff --git a/doc/build/errors.rst b/doc/build/errors.rst index 994638c0d..8c9554f9c 100644 --- a/doc/build/errors.rst +++ b/doc/build/errors.rst @@ -318,6 +318,77 @@ the database driver (DBAPI), not SQLAlchemy itself. SQL Expression Language ======================= +.. _error_l7de: + +Compiler StrSQLCompiler can't render element of type <element type> +------------------------------------------------------------------- + +This error usually occurs when attempting to stringify a SQL expression +construct that includes elements which are not part of the default compilation; +in this case, the error will be against the :class:`.StrSQLCompiler` class. +In less common cases, it can also occur when the wrong kind of SQL expression +is used with a particular type of database backend; in those cases, other +kinds of SQL compiler classes will be named, such as ``SQLCompiler`` or +``sqlalchemy.dialects.postgresql.PGCompiler``. The guidance below is +more specific to the "stringification" use case but describes the general +background as well. + +Normally, a Core SQL construct or ORM :class:`.Query` object can be stringified +directly, such as when we use ``print()``:: + + >>> from sqlalchemy import column + >>> print(column('x') == 5) + x = :x_1 + +When the above SQL expression is stringified, the :class:`.StrSQLCompiler` +compiler class is used, which is a special statement compiler that is invoked +when a construct is stringified without any dialect-specific information. + +However, there are many constructs that are specific to some particular kind +of database dialect, for which the :class:`.StrSQLCompiler` doesn't know how +to turn into a string, such as the PostgreSQL +`"insert on conflict" <postgresql_insert_on_conflict>`_ construct:: + + >>> from sqlalchemy.dialects.postgresql import insert + >>> from sqlalchemy import table, column + >>> my_table = table('my_table', column('x'), column('y')) + >>> insert_stmt = insert(my_table).values(x='foo') + >>> insert_stmt = insert_stmt.on_conflict_do_nothing( + ... index_elements=['y'] + ... ) + >>> print(insert_stmt) + Traceback (most recent call last): + + ... + + sqlalchemy.exc.UnsupportedCompilationError: + Compiler <sqlalchemy.sql.compiler.StrSQLCompiler object at 0x7f04fc17e320> + can't render element of type + <class 'sqlalchemy.dialects.postgresql.dml.OnConflictDoNothing'> + +In order to stringify constructs that are specific to particular backend, +the :meth:`.ClauseElement.compile` method must be used, passing either an +:class:`.Engine` or a :class:`.Dialect` object which will invoke the correct +compiler. Below we use a PostgreSQL dialect:: + + >>> from sqlalchemy.dialects import postgresql + >>> print(insert_stmt.compile(dialect=postgresql.dialect())) + INSERT INTO my_table (x) VALUES (%(x)s) ON CONFLICT (y) DO NOTHING + +For an ORM :class:`.Query` object, the statement can be accessed using the +:attr:`~.orm.query.Query.statement` accessor:: + + statement = query.statement + print(statement.compile(dialect=postgresql.dialect())) + +See the FAQ link below for additional detail on direct stringification / +compilation of SQL elements. + +.. seealso:: + + :ref:`faq_sql_expression_string` + + TypeError: <operator> not supported between instances of 'ColumnProperty' and <something> ----------------------------------------------------------------------------------------- |