Add "String fields and column length" sections to docs

Author: YuriiMotov

docs/tutorial/str-fields-and-column-length.md

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+# String fields and column length
+
+Some databases have a limit on the length of string columns (e.g., `VARCHAR(255)` in *MySQL*) and fail with an error if you try to create a string column without specifying a length.
+
+**SQLModel** handles this automatically depending on the database dialect you are using. 😎
+
+For databases that require a length for string columns, **SQLModel** will automatically set a default length (e.g., `255` for *MySQL*) if you do not specify one.
+
+{* ./docs_src/tutorial/str_fields_and_column_length/tutorial001_py310.py ln[4:6] hl[6] *}
+
+If you run this code with *MySQL*, **SQLModel** will create the `name` column as `VARCHAR(255)`:
+
+```sql
+CREATE TABLE hero (
+        id INTEGER NOT NULL AUTO_INCREMENT,
+        name VARCHAR(255) NOT NULL,
+        PRIMARY KEY (id)
+)
+```
+
+But you can always specify a custom length if needed:
+
+{* ./docs_src/tutorial/str_fields_and_column_length/tutorial002_py310.py ln[4:6] hl[6] *}
+
+```sql
+CREATE TABLE hero (
+        id INTEGER NOT NULL AUTO_INCREMENT,
+        name VARCHAR(100) NOT NULL,
+        PRIMARY KEY (id)
+)
+```
+This works thanks to `AutoString` type that **SQLModel** uses for all string fields by default.
+
+But if you specify the database type of column explicitly, **SQLModel** will not be able to set the length automatically, and you will need to specify it manually:
+
+{* ./docs_src/tutorial/str_fields_and_column_length/tutorial003_py310.py ln[1:6] hl[1,6] *}
+
+The code example above will fail on databases that require a length for string columns:
+
+```console
+sqlalchemy.exc.CompileError: (in table 'hero', column 'name'): VARCHAR requires a length on dialect mysql
+```
+
+To fix it, you need to specify the length explicitly as follows:
+
+{* ./docs_src/tutorial/str_fields_and_column_length/tutorial004_py310.py ln[1:6] hl[1,6] *}
+
+This will give:
+
+```sql
+CREATE TABLE hero (
+        id INTEGER NOT NULL AUTO_INCREMENT,
+        name VARCHAR(255) NOT NULL,
+        PRIMARY KEY (id)
+)
+```

docs_src/tutorial/str_fields_and_column_length/__init__.py

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+from sqlmodel import Field, SQLModel, create_engine
+
+
+class Hero(SQLModel, table=True):
+    id: int | None = Field(default=None, primary_key=True)
+    name: str
+
+
+database_url = "mysql://user:password@localhost/dbname"
+
+engine = create_engine(database_url, echo=True)
+
+
+def create_db_and_tables():
+    SQLModel.metadata.create_all(engine)
+
+
+if __name__ == "__main__":
+    create_db_and_tables()

docs_src/tutorial/str_fields_and_column_length/tutorial001_py310.py

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+from typing import Optional
+
+from sqlmodel import Field, SQLModel, create_engine
+
+
+class Hero(SQLModel, table=True):
+    id: Optional[int] = Field(default=None, primary_key=True)
+    name: str
+
+
+database_url = "mysql://user:password@localhost/dbname"
+
+engine = create_engine(database_url, echo=True)
+
+
+def create_db_and_tables():
+    SQLModel.metadata.create_all(engine)
+
+
+if __name__ == "__main__":
+    create_db_and_tables()

docs_src/tutorial/str_fields_and_column_length/tutorial001_py39.py

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+from sqlmodel import Field, SQLModel, create_engine
+
+
+class Hero(SQLModel, table=True):
+    id: int | None = Field(default=None, primary_key=True)
+    name: str = Field(max_length=100)
+
+
+database_url = "mysql://user:password@localhost/dbname"
+
+engine = create_engine(database_url, echo=True)
+
+
+def create_db_and_tables():
+    SQLModel.metadata.create_all(engine)
+
+
+if __name__ == "__main__":
+    create_db_and_tables()

docs_src/tutorial/str_fields_and_column_length/tutorial002_py310.py

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+from typing import Optional
+
+from sqlmodel import Field, SQLModel, create_engine
+
+
+class Hero(SQLModel, table=True):
+    id: Optional[int] = Field(default=None, primary_key=True)
+    name: str = Field(max_length=100)
+
+
+database_url = "mysql://user:password@localhost/dbname"
+
+engine = create_engine(database_url, echo=True)
+
+
+def create_db_and_tables():
+    SQLModel.metadata.create_all(engine)
+
+
+if __name__ == "__main__":
+    create_db_and_tables()

docs_src/tutorial/str_fields_and_column_length/tutorial002_py39.py

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+from sqlmodel import Field, SQLModel, String, create_engine
+
+
+class Hero(SQLModel, table=True):
+    id: int | None = Field(default=None, primary_key=True)
+    name: str = Field(sa_type=String)
+
+
+database_url = "mysql://user:password@localhost/dbname"
+
+engine = create_engine(database_url, echo=True)
+
+
+def create_db_and_tables():
+    SQLModel.metadata.create_all(engine)
+
+
+if __name__ == "__main__":
+    create_db_and_tables()

docs_src/tutorial/str_fields_and_column_length/tutorial003_py310.py

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+from typing import Optional
+
+from sqlmodel import Field, SQLModel, String, create_engine
+
+
+class Hero(SQLModel, table=True):
+    id: Optional[int] = Field(default=None, primary_key=True)
+    name: str = Field(sa_type=String)
+
+
+database_url = "mysql://user:password@localhost/dbname"
+
+engine = create_engine(database_url, echo=True)
+
+
+def create_db_and_tables():
+    SQLModel.metadata.create_all(engine)
+
+
+if __name__ == "__main__":
+    create_db_and_tables()

docs_src/tutorial/str_fields_and_column_length/tutorial003_py39.py

@@ -0,0 +1,19 @@
+from sqlmodel import Field, SQLModel, String, create_engine
+
+
+class Hero(SQLModel, table=True):
+    id: int | None = Field(default=None, primary_key=True)
+    name: str = Field(sa_type=String(length=255))
+
+
+database_url = "mysql://user:password@localhost/dbname"
+
+engine = create_engine(database_url, echo=True)
+
+
+def create_db_and_tables():
+    SQLModel.metadata.create_all(engine)
+
+
+if __name__ == "__main__":
+    create_db_and_tables()

docs_src/tutorial/str_fields_and_column_length/tutorial004_py310.py

@@ -0,0 +1,21 @@
+from typing import Optional
+
+from sqlmodel import Field, SQLModel, String, create_engine
+
+
+class Hero(SQLModel, table=True):
+    id: Optional[int] = Field(default=None, primary_key=True)
+    name: str = Field(sa_type=String(length=255))
+
+
+database_url = "mysql://user:password@localhost/dbname"
+
+engine = create_engine(database_url, echo=True)
+
+
+def create_db_and_tables():
+    SQLModel.metadata.create_all(engine)
+
+
+if __name__ == "__main__":
+    create_db_and_tables()