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112 lines
5.5 KiB
Markdown
112 lines
5.5 KiB
Markdown
Field Options
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=============
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All field types accept the following arguments:
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- default
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- alias
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- materialized
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- readonly
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- codec
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Note that `default`, `alias` and `materialized` are mutually exclusive - you cannot use more than one of them in a single field.
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## default
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Specifies a default value to use for the field. If not given, the field will have a default value based on its type: empty string for string fields, zero for numeric fields, etc.
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The default value can be a Python value suitable for the field type, or an expression. For example:
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```python
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class Event(Model):
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name = StringField(default="EVENT")
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repeated = UInt32Field(default=1)
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created = DateTimeField(default=F.now())
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engine = Memory()
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...
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```
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When creating a model instance, any fields you do not specify get their default value. Fields that use a default expression are assigned a sentinel value of `infi.clickhouse_orm.utils.NO_VALUE` instead. For example:
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```python
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>>> event = Event()
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>>> print(event.to_dict())
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{'name': 'EVENT', 'repeated': 1, 'created': <NO_VALUE>}
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```
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:warning: Due to a bug in ClickHouse versions prior to 20.1.2.4, insertion of records with expressions for default values may fail.
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## alias / materialized
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The `alias` and `materialized` attributes expect an expression that gets calculated by the database. The difference is that `alias` fields are calculated on the fly, while `materialized` fields are calculated when the record is inserted, and are stored on disk.
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You can use any expression, and can refer to other model fields. For example:
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```python
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class Event(Model):
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created = DateTimeField()
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created_date = DateTimeField(materialized=F.toDate(created))
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name = StringField()
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normalized_name = StringField(alias=F.upper(F.trim(name)))
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engine = Memory()
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```
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For backwards compatibility with older versions of the ORM, you can pass the expression as an SQL string:
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```python
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created_date = DateTimeField(materialized="toDate(created)")
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```
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Both field types can't be inserted into the database directly, so they are ignored when using the `Database.insert()` method. ClickHouse does not return the field values if you use `"SELECT * FROM ..."` - you have to list these field names explicitly in the query.
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Usage:
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```python
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obj = Event(created=datetime.now(), name='MyEvent')
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db = Database('my_test_db')
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db.insert([obj])
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# All values will be retrieved from database
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db.select('SELECT created, created_date, username, name FROM $db.event', model_class=Event)
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# created_date and username will contain a default value
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db.select('SELECT * FROM $db.event', model_class=Event)
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```
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When creating a model instance, any alias or materialized fields are assigned a sentinel value of `infi.clickhouse_orm.utils.NO_VALUE` since their real values can only be known after insertion to the database.
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## codec
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This attribute specifies the compression algorithm to use for the field (instead of the default data compression algorithm defined in server settings).
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Supported compression algorithms:
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| Codec | Argument | Comment
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| -------------------- | -------------------------------------------| ----------------------------------------------------
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| NONE | None | No compression.
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| LZ4 | None | LZ4 compression.
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| LZ4HC(`level`) | Possible `level` range: [3, 12]. | Default value: 9. Greater values stands for better compression and higher CPU usage. Recommended value range: [4,9].
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| ZSTD(`level`) | Possible `level`range: [1, 22]. | Default value: 1. Greater values stands for better compression and higher CPU usage. Levels >= 20, should be used with caution, as they require more memory.
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| Delta(`delta_bytes`) | Possible `delta_bytes` range: 1, 2, 4 , 8. | Default value for `delta_bytes` is `sizeof(type)` if it is equal to 1, 2,4 or 8 and equals to 1 otherwise.
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Codecs can be combined by separating their names with commas. The default database codec is not included into pipeline (if it should be applied to a field, you have to specify it explicitly in pipeline).
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Recommended usage for codecs:
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- When values for particular metric do not differ significantly from point to point, delta-encoding allows to reduce disk space usage significantly.
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- DateTime works great with pipeline of Delta, ZSTD and the column size can be compressed to 2-3% of its original size (given a smooth datetime data)
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- Numeric types usually enjoy best compression rates with ZSTD
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- String types enjoy good compression rates with LZ4HC
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Example:
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```python
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class Stats(Model):
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id = UInt64Field(codec='ZSTD(10)')
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timestamp = DateTimeField(codec='Delta,ZSTD')
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timestamp_date = DateField(codec='Delta(4),ZSTD(22)')
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metadata_id = Int64Field(codec='LZ4')
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status = StringField(codec='LZ4HC(10)')
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calculation = NullableField(Float32Field(), codec='ZSTD')
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alerts = ArrayField(FixedStringField(length=15), codec='Delta(2),LZ4HC')
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engine = MergeTree('timestamp_date', ('id', 'timestamp'))
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```
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Note: This feature is supported on ClickHouse version 19.1.16 and above. Codec arguments will be ignored by the ORM for older versions of ClickHouse.
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## readonly
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This attribute is set automatically for fields with `alias` or `materialized` attributes, you do not need to pass it yourself.
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---
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[<< Querysets](querysets.md) | [Table of Contents](toc.md) | [Field Types >>](field_types.md) |