diff --git a/CHANGELOG.md b/CHANGELOG.md
index da7d3c4..b6165b4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,10 @@
 Change Log
 ==========
 
+Unreleased
+----------
+- Support for model constraints
+
 v2.0.1
 ------
 - Remove unnecessary import of `six`
diff --git a/docs/class_reference.md b/docs/class_reference.md
index 285f9b4..616863f 100644
--- a/docs/class_reference.md
+++ b/docs/class_reference.md
@@ -178,7 +178,7 @@ Unrecognized field names will cause an `AttributeError`.
 #### Model.create_table_sql(db)
 
 
-Returns the SQL command for creating a table for this model.
+Returns the SQL statement for creating a table for this model.
 
 
 #### Model.drop_table_sql(db)
@@ -308,7 +308,7 @@ Unrecognized field names will cause an `AttributeError`.
 #### BufferModel.create_table_sql(db)
 
 
-Returns the SQL command for creating a table for this model.
+Returns the SQL statement for creating a table for this model.
 
 
 #### BufferModel.drop_table_sql(db)
@@ -422,12 +422,147 @@ Returns the instance's column values as a tab-separated line. A newline is not i
 - `include_readonly`: if false, returns only fields that can be inserted into database.
 
 
+### MergeModel
+
+Extends Model
+
+
+Model for Merge engine
+Predefines virtual _table column an controls that rows can't be inserted to this table type
+https://clickhouse.tech/docs/en/single/index.html#document-table_engines/merge
+
+#### MergeModel(**kwargs)
+
+
+Creates a model instance, using keyword arguments as field values.
+Since values are immediately converted to their Pythonic type,
+invalid values will cause a `ValueError` to be raised.
+Unrecognized field names will cause an `AttributeError`.
+
+
+#### MergeModel.create_table_sql(db)
+
+
+Returns the SQL statement for creating a table for this model.
+
+
+#### MergeModel.drop_table_sql(db)
+
+
+Returns the SQL command for deleting this model's table.
+
+
+#### MergeModel.fields(writable=False)
+
+
+Returns an `OrderedDict` of the model's fields (from name to `Field` instance).
+If `writable` is true, only writable fields are included.
+Callers should not modify the dictionary.
+
+
+#### MergeModel.from_tsv(line, field_names, timezone_in_use=UTC, database=None)
+
+
+Create a model instance from a tab-separated line. The line may or may not include a newline.
+The `field_names` list must match the fields defined in the model, but does not have to include all of them.
+
+- `line`: the TSV-formatted data.
+- `field_names`: names of the model fields in the data.
+- `timezone_in_use`: the timezone to use when parsing dates and datetimes.
+- `database`: if given, sets the database that this instance belongs to.
+
+
+#### get_database()
+
+
+Gets the `Database` that this model instance belongs to.
+Returns `None` unless the instance was read from the database or written to it.
+
+
+#### get_field(name)
+
+
+Gets a `Field` instance given its name, or `None` if not found.
+
+
+#### MergeModel.has_funcs_as_defaults()
+
+
+Return True if some of the model's fields use a function expression
+as a default value. This requires special handling when inserting instances.
+
+
+#### MergeModel.is_read_only()
+
+
+Returns true if the model is marked as read only.
+
+
+#### MergeModel.is_system_model()
+
+
+Returns true if the model represents a system table.
+
+
+#### MergeModel.objects_in(database)
+
+
+Returns a `QuerySet` for selecting instances of this model class.
+
+
+#### set_database(db)
+
+
+Sets the `Database` that this model instance belongs to.
+This is done automatically when the instance is read from the database or written to it.
+
+
+#### MergeModel.table_name()
+
+
+Returns the model's database table name. By default this is the
+class name converted to lowercase. Override this if you want to use
+a different table name.
+
+
+#### to_db_string()
+
+
+Returns the instance as a bytestring ready to be inserted into the database.
+
+
+#### to_dict(include_readonly=True, field_names=None)
+
+
+Returns the instance's column values as a dict.
+
+- `include_readonly`: if false, returns only fields that can be inserted into database.
+- `field_names`: an iterable of field names to return (optional)
+
+
+#### to_tskv(include_readonly=True)
+
+
+Returns the instance's column keys and values as a tab-separated line. A newline is not included.
+Fields that were not assigned a value are omitted.
+
+- `include_readonly`: if false, returns only fields that can be inserted into database.
+
+
+#### to_tsv(include_readonly=True)
+
+
+Returns the instance's column values as a tab-separated line. A newline is not included.
+
+- `include_readonly`: if false, returns only fields that can be inserted into database.
+
+
 ### DistributedModel
 
 Extends Model
 
 
-Model for Distributed engine
+Model class for use with a `Distributed` engine.
 
 #### DistributedModel(**kwargs)
 
@@ -441,6 +576,9 @@ Unrecognized field names will cause an `AttributeError`.
 #### DistributedModel.create_table_sql(db)
 
 
+Returns the SQL statement for creating a table for this model.
+
+
 #### DistributedModel.drop_table_sql(db)
 
 
@@ -541,6 +679,10 @@ Returns a `QuerySet` for selecting instances of this model class.
 #### set_database(db)
 
 
+Sets the `Database` that this model instance belongs to.
+This is done automatically when the instance is read from the database or written to it.
+
+
 #### DistributedModel.table_name()
 
 
@@ -581,6 +723,26 @@ Returns the instance's column values as a tab-separated line. A newline is not i
 - `include_readonly`: if false, returns only fields that can be inserted into database.
 
 
+### Constraint
+
+
+Defines a model constraint.
+
+#### Constraint(expr)
+
+
+Initializer. Requires an expression that ClickHouse will verify when inserting data.
+
+
+#### create_table_sql()
+
+
+Returns the SQL statement for defining this constraint on table creation.
+
+
+#### str()
+
+
 infi.clickhouse_orm.fields
 --------------------------
 
diff --git a/docs/models_and_databases.md b/docs/models_and_databases.md
index b1f262c..928489d 100644
--- a/docs/models_and_databases.md
+++ b/docs/models_and_databases.md
@@ -75,6 +75,25 @@ The table name used for the model is its class name, converted to lowercase. To
         def table_name(cls):
             return 'people'
 
+### Model Constraints
+
+It is possible to define constraints which ClickHouse verifies when data is inserted. Trying to insert invalid records will raise a `ServerError`. Each constraint has a name and an expression to validate. For example:
+
+    from infi.clickhouse_orm import Model, Constraint, F, StringField, DateField, Float32Field, MergeTree
+
+    class Person(Model):
+
+        first_name = StringField()
+        last_name = StringField()
+        birthday = DateField()
+        height = Float32Field()
+
+        # Ensure that the birthday is not a future date
+        birthday_is_in_the_past = Constraint(birthday <= F.today())
+
+        engine = MergeTree('birthday', ('first_name', 'last_name', 'birthday'))
+
+
 Using Models
 ------------
 
diff --git a/docs/schema_migrations.md b/docs/schema_migrations.md
index 1556395..9e3fa01 100644
--- a/docs/schema_migrations.md
+++ b/docs/schema_migrations.md
@@ -33,19 +33,19 @@ Each migration file is expected to contain a list of `operations`, for example:
 The following operations are supported:
 
 
-**CreateTable**
+### CreateTable
 
 A migration operation that creates a table for a given model class. If the table already exists, the operation does nothing.
 
 In case the model class is a `BufferModel`, the operation first creates the underlying on-disk table, and then creates the buffer table.
 
 
-**DropTable**
+### DropTable
 
 A migration operation that drops the table of a given model class. If the table does not exist, the operation does nothing.
 
 
-**AlterTable**
+### AlterTable
 
 A migration operation that compares the table of a given model class to the model’s fields, and alters the table to match the model. The operation can:
 
@@ -56,14 +56,19 @@ A migration operation that compares the table of a given model class to the mode
 Default values are not altered by this operation.
 
 
-**AlterTableWithBuffer**
+### AlterTableWithBuffer
 
 A compound migration operation for altering a buffer table and its underlying on-disk table. The buffer table is dropped, the on-disk table is altered, and then the buffer table is re-created. This is the procedure recommended in the ClickHouse documentation for handling scenarios in which the underlying table needs to be modified.
 
 Applying this migration operation to a regular table has the same effect as an `AlterTable` operation.
 
 
-**RunPython**
+### AlterConstraints
+
+A migration operation that adds new constraints from the model to the database table, and drops obsolete ones. Constraints are identified by their names, so a change in an existing constraint will not be detected unless its name was changed too. ClickHouse does not check that the constraints hold for existing data in the table.
+
+
+### RunPython
 
 A migration operation that runs a Python function. The function receives the `Database` instance to operate on.
 
@@ -77,9 +82,9 @@ A migration operation that runs a Python function. The function receives the `Da
     ]
 
 
-**RunSQL**
+### RunSQL
 
-A migration operation that runs raw SQL queries. It expects a string containing an SQL query, or an array of SQL-query strings.
+A migration operation that runs raw SQL statements. It expects a string containing an SQL statements, or a list of statements.
 
 Example:
 
diff --git a/docs/toc.md b/docs/toc.md
index 5805eaa..6dd5da0 100644
--- a/docs/toc.md
+++ b/docs/toc.md
@@ -10,6 +10,7 @@
          * [Materialized fields](models_and_databases.md#materialized-fields)
          * [Alias fields](models_and_databases.md#alias-fields)
          * [Table Names](models_and_databases.md#table-names)
+         * [Model Constraints](models_and_databases.md#model-constraints)
       * [Using Models](models_and_databases.md#using-models)
       * [Inserting to the Database](models_and_databases.md#inserting-to-the-database)
       * [Reading from the Database](models_and_databases.md#reading-from-the-database)
@@ -58,6 +59,13 @@
 
    * [Schema Migrations](schema_migrations.md#schema-migrations)
       * [Writing Migrations](schema_migrations.md#writing-migrations)
+         * [CreateTable](schema_migrations.md#createtable)
+         * [DropTable](schema_migrations.md#droptable)
+         * [AlterTable](schema_migrations.md#altertable)
+         * [AlterTableWithBuffer](schema_migrations.md#altertablewithbuffer)
+         * [AlterConstraints](schema_migrations.md#alterconstraints)
+         * [RunPython](schema_migrations.md#runpython)
+         * [RunSQL](schema_migrations.md#runsql)
       * [Running Migrations](schema_migrations.md#running-migrations)
 
    * [System Models](system_models.md#system-models)
@@ -74,7 +82,9 @@
       * [infi.clickhouse_orm.models](class_reference.md#inficlickhouse_ormmodels)
          * [Model](class_reference.md#model)
          * [BufferModel](class_reference.md#buffermodel)
+         * [MergeModel](class_reference.md#mergemodel)
          * [DistributedModel](class_reference.md#distributedmodel)
+         * [Constraint](class_reference.md#constraint)
       * [infi.clickhouse_orm.fields](class_reference.md#inficlickhouse_ormfields)
          * [ArrayField](class_reference.md#arrayfield)
          * [BaseEnumField](class_reference.md#baseenumfield)
diff --git a/scripts/generate_ref.py b/scripts/generate_ref.py
index 6e537ec..8dc0477 100644
--- a/scripts/generate_ref.py
+++ b/scripts/generate_ref.py
@@ -132,7 +132,7 @@ if __name__ == '__main__':
     print('===============')
     print()
     module_doc([database.Database, database.DatabaseException])
-    module_doc([models.Model, models.BufferModel, models.DistributedModel])
+    module_doc([models.Model, models.BufferModel, models.MergeModel, models.DistributedModel, models.Constraint])
     module_doc(sorted([fields.Field] + all_subclasses(fields.Field), key=lambda x: x.__name__), False)
     module_doc([engines.Engine] + all_subclasses(engines.Engine), False)
     module_doc([query.QuerySet, query.AggregateQuerySet, query.Q])
diff --git a/src/infi/clickhouse_orm/migrations.py b/src/infi/clickhouse_orm/migrations.py
index cf93d9a..5361b2b 100644
--- a/src/infi/clickhouse_orm/migrations.py
+++ b/src/infi/clickhouse_orm/migrations.py
@@ -7,7 +7,7 @@ import logging
 logger = logging.getLogger('migrations')
 
 
-class Operation(object):
+class Operation():
     '''
     Base class for migration operations.
     '''
@@ -16,14 +16,31 @@ class Operation(object):
         raise NotImplementedError()   # pragma: no cover
 
 
-class CreateTable(Operation):
+class ModelOperation(Operation):
     '''
-    A migration operation that creates a table for a given model class.
+    Base class for migration operations that work on a specific model.
     '''
 
     def __init__(self, model_class):
+        '''
+        Initializer.
+        '''
         self.model_class = model_class
 
+    def _alter_table(self, database, cmd):
+        '''
+        Utility for running ALTER TABLE commands.
+        '''
+        cmd = "ALTER TABLE $db.`%s` %s" % (self.model_class.table_name(), cmd)
+        logger.debug(cmd)
+        database.raw(cmd)
+
+
+class CreateTable(ModelOperation):
+    '''
+    A migration operation that creates a table for a given model class.
+    '''
+
     def apply(self, database):
         logger.info('    Create table %s', self.model_class.table_name())
         if issubclass(self.model_class, BufferModel):
@@ -31,7 +48,7 @@ class CreateTable(Operation):
         database.create_table(self.model_class)
 
 
-class AlterTable(Operation):
+class AlterTable(ModelOperation):
     '''
     A migration operation that compares the table of a given model class to
     the model's fields, and alters the table to match the model. The operation can:
@@ -41,18 +58,10 @@ class AlterTable(Operation):
     Default values are not altered by this operation.
     '''
 
-    def __init__(self, model_class):
-        self.model_class = model_class
-
     def _get_table_fields(self, database):
         query = "DESC `%s`.`%s`" % (database.db_name, self.model_class.table_name())
         return [(row.name, row.type) for row in database.select(query)]
 
-    def _alter_table(self, database, cmd):
-        cmd = "ALTER TABLE `%s`.`%s` %s" % (database.db_name, self.model_class.table_name(), cmd)
-        logger.debug(cmd)
-        database._send(cmd)
-
     def apply(self, database):
         logger.info('    Alter table %s', self.model_class.table_name())
 
@@ -100,16 +109,13 @@ class AlterTable(Operation):
                 self._alter_table(database, 'MODIFY COLUMN %s %s' % (field_name, model_fields[field_name]))
 
 
-class AlterTableWithBuffer(Operation):
+class AlterTableWithBuffer(ModelOperation):
     '''
     A migration operation for altering a buffer table and its underlying on-disk table.
     The buffer table is dropped, the on-disk table is altered, and then the buffer table
     is re-created.
     '''
 
-    def __init__(self, model_class):
-        self.model_class = model_class
-
     def apply(self, database):
         if issubclass(self.model_class, BufferModel):
             DropTable(self.model_class).apply(database)
@@ -119,25 +125,60 @@ class AlterTableWithBuffer(Operation):
             AlterTable(self.model_class).apply(database)
 
 
-class DropTable(Operation):
+class DropTable(ModelOperation):
     '''
     A migration operation that drops the table of a given model class.
     '''
 
-    def __init__(self, model_class):
-        self.model_class = model_class
-
     def apply(self, database):
         logger.info('    Drop table %s', self.model_class.table_name())
         database.drop_table(self.model_class)
 
 
+class AlterConstraints(ModelOperation):
+    '''
+    A migration operation that adds new constraints from the model to the database
+    table, and drops obsolete ones. Constraints are identified by their names, so
+    a change in an existing constraint will not be detected unless its name was changed too.
+    ClickHouse does not check that the constraints hold for existing data in the table.
+    '''
+
+    def apply(self, database):
+        logger.info('    Alter constraints for %s', self.model_class.table_name())
+        existing = self._get_constraint_names(database)
+        # Go over constraints in the model
+        for constraint in self.model_class._constraints.values():
+            # Check if it's a new constraint
+            if constraint.name not in existing:
+                logger.info('        Add constraint %s', constraint.name)
+                self._alter_table(database, 'ADD %s' % constraint.create_table_sql())
+            else:
+                existing.remove(constraint.name)
+        # Remaining constraints in `existing` are obsolete
+        for name in existing:
+            logger.info('        Drop constraint %s', name)
+            self._alter_table(database, 'DROP CONSTRAINT `%s`' % name)
+
+    def _get_constraint_names(self, database):
+        '''
+        Returns a set containing the names of existing constraints in the table.
+        '''
+        import re
+        create_table_sql = database.raw('SHOW CREATE TABLE $db.`%s`' % self.model_class.table_name())
+        matches = re.findall(r'\sCONSTRAINT\s+`?(.+?)`?\s+CHECK\s', create_table_sql, flags=re.IGNORECASE)
+        return set(matches)
+
+
 class RunPython(Operation):
     '''
-    A migration operation that executes given python function on database
+    A migration operation that executes a Python function.
     '''
     def __init__(self, func):
-        assert callable(func), "'func' parameter must be function"
+        '''
+        Initializer. The given Python function will be called with a single
+        argument - the Database instance to apply the migration to.
+        '''
+        assert callable(func), "'func' argument must be function"
         self._func = func
 
     def apply(self, database):
@@ -147,14 +188,17 @@ class RunPython(Operation):
 
 class RunSQL(Operation):
     '''
-    A migration operation that executes given SQL on database
+    A migration operation that executes arbitrary SQL statements.
     '''
 
     def __init__(self, sql):
+        '''
+        Initializer. The given sql argument must be a valid SQL statement or
+        list of statements.
+        '''
         if isinstance(sql, str):
             sql = [sql]
-
-        assert isinstance(sql, list), "'sql' parameter must be string or list of strings"
+        assert isinstance(sql, list), "'sql' argument must be string or list of strings"
         self._sql = sql
 
     def apply(self, database):